BMG Direct Marketing, Inc. v. Peake (178 S.W.3d 763, 2005)

Justice O’NEILL

delivered the opinion of the Court,

in which Chief Justice JEFFERSON, Justice HECHT, Justice BRISTER, Justice MEDINA, Justice GREEN, Justice JOHNSON, and Justice WILLETT joined.

In this class-action case, the trial court certified a class of music club members who paid allegedly illegal late fees for failing to timely pay for compact discs in accordance with the terms of a membership agreement. The parties dispute whether the customers’ claims are subject to the voluntary-payment rule, which has sometimes been successfully invoked to bar restitution of money that has been voluntarily paid with full knowledge of all the facts and without fraud, deception, duress, or coercion. Although the voluntary-payment rule’s application has been largely supplanted by the creation of legal and statutory remedies, we conclude that the rule applies to the customers’ claims for restitution of late fees in this case. Because the trial court failed to analyze the rule’s effect on the requirements for class certification and how the claims in this case would be tried, we decertify the class and remand the case to the trial court for proceedings consistent with this opinion.

I. Background

BMG Direct Marketing operates music clubs that sell compact discs to club members, principally through direct mail and online services. Membership in a BMG club begins with one of a variety of special promotions, typically “11 CDs for the price of one” or “12 CDs for the price of one.” BMG assesses a late fee of $1.50 if club members do not pay for the compact discs within thirty days. All BMG promotions include a statement that late fees will apply to past-due BMG invoices. Each customer’s initial shipment of compact discs includes a Membership Guide which provides: “Payment is due when you receive your shipment. Late charges will be added to your account for amounts unpaid after 30 days.” In addition, each BMG shipment, including the initial one, contains an invoice which specifies: “If payment is not made within 30 days, a late charge of $1.50 will be added to your account (not a finance charge).” Customers are given ten days to decline membership for any reason and may return the compact discs at BMG’s expense with no further obligation.

Named plaintiff Patrick Peake was a BMG club member who bought dozens of compact discs from the company from 1999 to 2002. During this period he incurred and paid to BMG late fees totaling $7.35. In 2002, Peake sued BMG to recover the late fees he had paid, claiming they constituted an illegal penalty because the fee charged did not reasonably forecast BMG’s actual damages resulting from customers’ late payments.

Peake moved to certify a class consisting of all present and former BMG club members in Texas who had paid BMG late fees since May 16, 1998. BMG opposed the motion, arguing that the voluntary-payment rule applied to each potential class member’s claims and precluded a finding that common issues would predominate. The trial court certified the class, noting that it was “unlikely” the voluntary-payment yule would apply in this case because the rule is equitable and “need not be applied where the rationale for its existence does not exist.” The trial court further held that, even if the voluntary-payment rule did apply, the “issue is not individual and may be determined as another common issue on a class wide basis.” A divided court of appeals affirmed without deciding whether or not the rule applied, holding that in either event the class members’ lack of full knowledge based upon BMG’s failure to disclose all material facts in the agreement would be a question common to the entire class. 175 S.W.3d 267, 269. We granted BMG’s petition to consider the voluntary-payment rule’s application to this case.

II. Liquidated Damages

Peake claims that BMG’s late fees constitute an unlawful penalty. In Texas, we distinguish a permissible liquidated damages clause from an unenforceable penalty. Phillips v. Phillips, 820 S.W.2d 785, 788 (Tex.1991). For such a clause to be enforceable, the fee charged must be a reasonable estimate of damages, and those damages must be incapable of precise calculation. Id. Companies enter into late-fee agreements with their customers because the precise damages that will result from their customers’ untimely payments is generally difficult if not impossible to' ascertain. See 24 Samuel Williston & Richard A. Lord, Williston on Contracts § 65:1, at 229 (4th ed.2002) (observing that a “liquidated damages clause is designed to substitute a sum agreed upon by the parties for any actual damages suffered as a result of a breach”); see also Phillips, 820 S.W.2d at 788 (“[T]o be enforceable as liquidated damages the damages must be uncertain....”). In this way, parties allocate the risk of uncertainty over the actual loss that will be realized if a customer’s payment is not timely. By entering into these contractual arrangements, “the need for the nonbreaching party to prove actual damages” is obviated. Williston & Lord, supra, § 65:1, at 280 (quoting Bair v. Axiom Design, L.L.C., 20 P.3d 388 (Utah 2001)). The agreed upon late fee thus quantifies a level of uncertainty — that both parties recognize — and allocates the risk of that uncertainty between the contracting parties.

The uncertainty inherent in calculating damages attributable to customers’ untimely payments is aptly demonstrated in the case law through the varied testimony of battling experts. Allowing parties to contractually allocate this risk of uncertainty carries out Texas’s public policy strongly favoring the freedom of parties to contract. See, e.g., In re Prudential Ins. Co. of Am., 148 S.W.3d 124, 129 (Tex.2004) (“As a rule, parties have the right to contract as they see fit as long as their agreement does not violate the law or public policy.”); Wood Motor Co. v. Nebel, 150 Tex. 86, 238 S.W.2d 181, 185 (1951). As we have said:

[I]f there is one thing which more than another public policy requires it is that men of full age and competent understanding shall have the utmost liberty of contracting, and that their contracts when entered into freely and voluntarily shall be held sacred and shall be enforced by Courts of justice. Therefore, you have this paramount public policy to consider — that you are not lightly to interfere with this freedom of contract.

Wood Motor Co., 238 S.W.2d at 185 (quoting Printing & Numerical Registering Co. v. Sampson, 19 L.R.Eq. 462, 465 (1875)).

Although parties may contractually allocate the risk of uncertainty over the amount of damages that will be incurred in the event of untimely payment, liquidated damages still must be a reasonable estimate of damages in order to be enforceable. In this case, Peake paid BMG’s late fees without protest and only later asserted that they were unenforceable; under these circumstances, BMG contends, Peake’s claims (and those of the class) are subject to the voluntary-payment defense. Before addressing this argument, we examine the voluntary-payment doctrine’s underpinnings and its development in our jurisprudence.

III. History of the Voluntary-Payment Rule

More than fifty years ago, we stated the common-law voluntary-payment rule as follows: “ ‘[M]oney voluntarily paid on a claim of right, with full knowledge of all the facts, in the absence of fraud, deception, duress, or compulsion, cannot be recovered back merely because the party . at the time of payment was ignorant of or mistook the law as to his liability.’ ” Pennell v. United Ins. Co., 150 Tex. 541, 243 S.W.2d 572, 576 (1951) (quoting 40 Am.Jur. § 205 (1942)). The rule is a defense to claims asserting unjust enrichment; that is, when a plaintiff sues for restitution claiming a payment constitutes unjust enrichment, a defendant may respond with the voluntary-payment rule as a defense. See Randazzo v. Harris Bank Palatine, N.A., 262 F.3d 663, 670 (7th Cir.2001) (noting that exceptions to the voluntary-payment rule are designed “to foster the resti-tutionary principles that have long stood in tension with the voluntary payment rule”); Restatement (Third) of Restitution & Unjust Enrichment § 6 cmt. e (Tentative Draft No. 1, 2001) (“The restitution claim to recover a payment in excess of an underlying liability ... meets an important limitation in the so-called voluntary-payment rule.”); see, e.g., Brown v. Oaklawn Bank, 718 S.W.2d 678, 681 (Tex.1986) (affirming restitution and rejecting voluntary-payment defense as payee had not detrimentally relied on payment).

This Court recognized the rule as early as 1880 in Ladd v. Southern Cotton Press and Manufacturing Co., 53 Tex. 172 (1880). In that case, Ladd, a cotton buyer, sued to recover money he paid to Southern Cotton Press, claiming that the payments were made involuntarily, under duress, and without consideration. Ladd, 53 Tex. at 173-74. Ladd alleged duress arising from the defendant’s unlawful combination with others to charge an amount greater than the services rendered were reasonably worth or that, in some instances, were not even performed. We rejected Ladd’s assertion of duress, noting that the defendant’s business was “open to all who wished to engage in it” and that “any one engaging in it may prescribe the terms upon which he will transact it. All parties employing his services, knowing his terms, would be bound by them.” Id. at 193. We concluded that the voluntary-payment rule applied to bar Ladd’s claims, stating:

As it appears from the petition that the payments were made with full knowledge of all the facts and without fraud or deception, unless made under duress, the amount thus paid cannot be recovered back, although so much as exceeds the reasonable value of the services rendered was paid without consideration.

Id. at 192. There being no evidence of duress, we held that “[ajppellant certainly had ample time and opportunity to have had his day in court, before the business between him and appellee was closed by the last voluntary payment made by him. Not having complained until the late date at which this suit was brought, he cannot now be heard to complain.” Id. at 194.

When the voluntary-payment rule is applied between private parties, as in Ladd, the underlying public policy has been described as follows:

[A] party who pays a claim is deemed to have made his own decision that it is justly due. If he thinks otherwise, he should resist. He should not pay out his money, leading the other party to act as though the matter were closed, and then be in a position to change his mind and invoke the aid of the courts to get it back.

R.G. McClung Cotton Co. v. Cotton Concentration Co., 479 S.W.2d 733, 743 (Tex.Civ.App.-Dallas 1972, writ refd n.r.e.). We have acknowledged that public policy favors protecting the finality of payments when a person is aware of all the facts upon which the liability to make payment depends, and there is no fraud, deception, duress, or coercion involved. Because of these policy concerns, this Court and Texas courts of appeals have, at times, applied the voluntary-payment rule between private parties. See Pennell, 243 S.W.2d at 575-76 (disallowing an insurance company recovery of payments made under a mistake of law because “money voluntarily paid on a claim of right, with full knowledge of all the facts, in the absence of fraud, duress, or compulsion, cannot be recovered back merely because the party at the time of payment was ignorant of or mistook the law as to his liability”); Gilliam v. Alford, 69 Tex. 267, 6 S.W. 757, 759 (1887) (disallowing recovery of a “voluntary settlement” when “the money was paid under a mistake of law; for it is well settled that money paid under a mistake of law with respect to liability to make payment, but with full knowledge of all the facts on which the claim for payment is based, and on which the right to resist it depends, cannot be recovered”); Ladd, 53 Tex. at 192-94; see also Southwest Indus. Import & Export, Inc. v. Borneo Sumatra Trading Co., 666 S.W.2d 625 (Tex.App.-Houston [1st Dist.] 1984, writ refd n.r.e.) (discussing the rule in the context of sale of goods); R.G. McClung Cotton Co., 479 S.W.2d at 743-44 (discussing the rule in the context of sale of goods, but allowing recovery because payor never intended to surrender his claim for damages for breach of contract); Runcie v. Runcie, 407 S.W.2d 861 (Tex.Civ.App.-Amarillo 1966, writ refd n.r.e.) (discussing the rule in the context of payment of bank note); Am. Cas. & Life Ins. Co. v. Boyd, 394 S.W.2d 685 (Tex.Civ.App.-Tyler 1965, no writ) (discussing the rule in context of payment of insurance premiums); but see TCI Cablevision of Dallas, Inc. v. Owens, 8 S.W.3d 837 (Tex.App.-Beaumont 2000, pet. dism’d by agr.) (discussing the voluntary payment rule in the context of cable late fees but concluding “it is far from clear that the voluntary payment doctrine applies here”).

The voluntary-payment rule has also been applied, albeit infrequently, to prohibit recovery of illegal taxes paid to the sovereign. This Court has stated: “A person who voluntarily pays an illegal tax has no claim for its repayment” unless that person paid under duress. Austin Nat’l Bank v. Sheppard, 123 Tex. 272, 71 S.W.2d 242, 245-46 (1934). In City of Houston v. Feizer, 76 Tex. 365, 13 S.W. 266, 267-68 (1890), we refused recovery to a butcher who had paid taxes to the city that the city was not authorized to collect. There, we observed that the rule against recovery of voluntary payments “precludes the courts being occupied in undoing the arrangements of parties which they have voluntarily made, and into which they have not been drawn by fraud or accident, or by any excusable ignorance of their legal rights and liabilities.” Id. at 267; see also Nat’l Biscuit Co. v. State, 134 Tex. 293, 135 S.W.2d 687, 693-94 (1940) (allowing recovery of permit fees and franchise taxes paid to the state under laws which were later declared unconstitutional because money was paid under duress); Sheppard, 71 S.W.2d at 246 (allowing recovery of taxes paid when an asphalt company paid under implied duress); Galveston Gas Co. v. County of Galveston, 54 Tex. 287, 292 (1881) (allowing recovery when “the taxes having been paid under protest to prevent the sale and consequent cloud on title, the payment was so far compulsory as to allow of a recovery back, if sought with reasonable promptness”); County of Galveston v. Gorham, 49 Tex. 279, 302, 308 (1878) (holding that it was “not unconscionable” for the county to retain funds paid to it when the payment was made voluntarily “because it was without objection paid under a mistake of law ... and there was no mistake of fact in paying it, and no deceit, fraud, or compulsion used in collecting it, or in causing it to be paid”). In the taxation context, the voluntary-payment rule is intended to “prevent the taxing entity from using funds paid by taxpayers in a given budget year and subsequently being required to refund these amounts.” City of Laredo v. S. Tex. Nat’l Bank, 775 S.W.2d 729, 731 (Tex.App.-San Antonio 1989, writ denied); see also Salvaggio v. Houston Indep. Sch. Dist., 752 S.W.2d 189, 193 (Tex.App.-Houston [14th Dist.] 1988, writ denied) (“The policy behind the rule is to discourage litigation and to secure the taxing authority in the orderly conduct of its affairs.”).

Although the voluntary-payment rule has been applied, at times, in both the private and public contexts, other legal and statutory remedies have evolved over time to supplant the rulers application in many of these contexts. Like other equitable claims and defenses, an adequate legal remedy may render equitable claims of unjust enrichment and equitable defenses of voluntary-payment unavailable. See, e.g., Fortune Prod. Co. v. Conoco, Inc., 52 S.W.3d 671, 684 (Tex.2000) (holding unjust enrichment inapplicable when parties have express contract covering the subject matter of the parties’ dispute); Bexar Bldg. & Loan Ass’n v. Robinson, 78 Tex. 163, 14 S.W. 227, 228 (1890) (holding usury statute prevented voluntary-payment defense). For example, the Texas Tax Code now provides that a person may recover a voluntary payment of certain illegal taxes, as long as the person paid under protest. Tex. Tax Code § 112.051(a) (“If a person who is required to pay a tax or fee imposed by this title or collected by the comptroller under any law ... contends that the tax or fee is unlawful ... the person shall pay the amount claimed by the state, and if the person intends to bring suit under this subchapter, the person must submit with the payment a protest.”); see also McKesson Corp. v. Div. of Alcoholic Beverages & Tobacco, 496 U.S. 18, 22, 110 S.Ct. 2238, 110 L.Ed.2d 17 (1990) (“[I]f a State penalizes taxpayers for failure to remit their taxes in timely fashion, thus requiring them to pay first and obtain review of the tax’s validity later in a refund action, the Due Process Clause requires the State to afford taxpayers a meaningful opportunity to secure postpayment relief for taxes already paid pursuant to a tax scheme ultimately found unconstitutional.”). Likewise, the voluntary-payment rule is no longer outcome determinative when deciding whether a judgment voluntarily paid moots an appeal. Now, the payment of a judgment without an “expressed intent” to continue an appeal moots the appeal, but payment with such an expression does not. See Miga v. Jensen, 96 S.W.3d 207, 212 (Tex.2002); see also Riner v. Briargrove Park Prop. Own ers, Inc., 858 S.W.2d 870, 370-71 (Tex.1998); Cont'l Cas. Co. v. Huizar, 740 S.W.2d 429, 430 (Tex.1987) (holding that an appeal was properly dismissed as moot where appellant paid the judgment without duress); Highland Church of Christ v. Powell, 640 S.W.2d 235, 236-37 (Tex.1982) (holding that payment of judgment did not moot appeal when paid under duress); Employees Fin. Co. v. Lathram, 369 S.W.2d 927, 929 (Tex.1963); Cravens v. Wilson, 48 Tex. 321, 323-24 (Tex.1877). Furthermore, whether an insurer’s voluntary payment of a claim can be recovered or reimbursed is considered a matter of contract, and whether a bank may recover voluntary payments mistakenly made to depositors is now governed by chapter 4 of the Uniform Commercial Code.

Thus, although the voluntary-payment rule may have been widely used by parties and some Texas courts at one time, its scope has diminished as the rule’s equitable policy concerns have been addressed through statutory or other legal remedies. Indeed, this Court has affirmatively applied the rule only once in the last forty years, and that holding has itself been modified since. See Huizar, 740 S.W.2d at 430 (holding voluntary payment of judgment without duress, although under protest, mooted appeal); cf. Miga, 96 S.W.3d at 212 (holding payment of judgment will not moot appeal if accompanied by clear expression of intent to prosecute appeal). Nevertheless, we have never abrogated the voluntary-payment rule, and it still has limited application in Texas jurisprudence.

IV. The Voluntary-Payment Rule’s Application in this Case

As already noted, the trial court and court of appeals rejected BMG’s assertion that the voluntary-payment defense precluded certification on three grounds: (1) it was “unlikely” the defense would apply, (2) the rationale for it did not exist in this case, and (3) if it did apply, it could be decided on a class-wide basis. 175 S.W.3d at 268. As to the first two grounds, there is nothing in the record or Texas law to indicate that the voluntary-payment rule should not generally apply to a customer’s claim for restitution of a contractually agreed-upon late fee alleged to be an unenforceable illegal penalty, nor have other remedies supplanted the rule’s application in that context. As to the latter point, the trial court did not analyze the potential difficulties in applying the rule on a class-wide basis and we are unable to assess the issue on the record before us.

A. The Rule in Other Jurisdictions

Neither the voluntary-payment rule nor the type of class litigation involved here is unique to Texas. Citing its own decision in TCI Cablevision, 8 S.W.3d at 845, the court of appeals found a “split of authority” that made it “far from clear” whether the voluntary-payment rale would apply. 175 S.W.3d at 268. But the split of authority is mostly one-sided; indeed, the court of appeals stands almost alone. With one exception, courts in every other jurisdiction have applied the voluntary-payment rale to claims that a late fee like the one here is an illegal penalty. See Hill v. Galaxy Telecom, 2000 U.S. Dist. LEXIS 2404 at *6 (N.D.Miss.2000); Home, 119 F.Supp.2d at 630; Putnam, 649 N.W.2d at 631-33; Dillon v. U-A Columbia Cablevision, 292 A.D.2d 25, 740 N.Y.S.2d 396, 398 (N.Y.App.Div.2002); Telescripps Cable Co., 542 S.E.2d at 643; Hassen., 751 So.2d at 1290; McWethy, 988 P.2d at 358; see also Smith v. Prime Cable of Chicago, 213 Ill. Dec. 304, 658 N.E.2d at 1330, 1332-34 (holding voluntary payment barred class action alleging pay-per-view show lasted two hours rather than three as advertised).

On the other side, one court has held that the voluntary-payment rule did not apply because late fees were not paid voluntarily when customers faced the “duress” of losing their cable television service. Whiteman, 802 N.E.2d at 890-93. Texas law has always required more than this to constitute “duress” under the voluntary-payment rule. See Ladd, 53 Tex. at 193 (holding private company’s refusal to do business except on its own terms “cannot be held to be duress”). While this Court has never addressed the voluntary-payment rule’s application to late fees, we long ago applied the rule in a similar context. In Hirshfield v. Fort Worth National Bank, 83 Tex. 452,18 S.W. 743 (1892), a notary charged the maker of a note a $3.50 “protest fee” for demanding payment on a past-due note, when in fact the note was not yet due. Id. at 744. Applying the voluntary-payment rule, this Court held that a fee “paid by the plaintiff voluntarily, with full knowledge of all the facts, and, at most, only under a mistake of law, cannot be recovered back.” Id. at 746.

Contrary to the court of appeals’ opinion, we do not find a substantial split of authority in Texas or across the nation generally that would categorically bar application of the voluntary-payment rule to the circumstances presented here. Thus, the trial court erred in concluding that the rule was “unlikely” to apply in this case.

B. The Rule’s Requirements

Nor do we agree that the rationale behind the voluntary-payment rule can be categorically dismissed, as the trial court did here. The Wisconsin Supreme Court has explained that the voluntary-payment rule “allows entities that receive payment for services to rely upon these funds and to use them unfettered in future activities,” and it “operates as a means to settle disputes without litigation by requiring the party contesting the payment to notify the payee of its concerns. After such notification, a payee who has acted wrongfully can react to rectify the situation.” Putnam, 649 N.W.2d at 633. We, too, recognize the benefit of a rule that allows entities to rely on contractually agreed-upon late fees received from customers who, having failed to protest, appear to have willingly paid with full knowledge of all the facts and without fraud, deception, coercion, or duress, for “a person who receives payment from another without any protest from the payor should be allowed to rely on use of the funds without risking a subsequent demand for return of the payment.” Id. at 635. The voluntary-payment rule also encourages discourse, rather than litigation, between customers and private enterprises that charge late fees in the course of their business.

Of course, for the voluntary-payment rule to apply, a person must pay “with full knowledge of all the facts.” See, e.g., Ladd, 53 Tex. at 192; Tyler v. Tyler, 742 S.W.2d 740, 743 (Houston [14th Dist.] 1987, writ denied). Peake contends that to satisfy this requirement, BMG’s customers must have known that the late fees were illegal and unenforceable when they paid them. Peake contends that because BMG did not disclose how the late fee was calculated, he and the other club members did not know and could not have known whether or not the fees charged were penal in nature or were a reasonable forecast of BMG’s actual or estimated damages. Therefore, according to Peake, he and the other club members lacked “full knowledge of all the facts,” precluding the voluntary-payment rule’s application.

BMG, on the other hand, contends it fully disclosed its late-fee policy to club members. When it shipped CDs to members, BMG made them aware of the late-fee amount and the circumstances under which it would be imposed, and offered them the opportunity to return the items with no further obligation. According to BMG, this gave its customers “full knowledge of all the facts” sufficient to trigger the voluntary-payment rule’s application.

We agree with BMG that knowledge of a late fee’s amount and the circumstances under which it will be imposed is sufficient to charge one with “full knowledge of the facts” for purposes of the voluntary-payment rule’s application. Accord Dillon v. U-A Columbia Cablevision of Westchester, Inc., 100 N.Y.2d 525, 760 N.Y.S.2d 726, 790 N.E.2d 1155, 1156 (2003); Putnam, 649 N.W.2d at 634. To hold otherwise would pose substantial practical problems. As the Wisconsin Supreme Court observed, accepting the argument that the voluntary-payment rule’s “full knowledge” requirement means a company has to disclose to customers precisely how its late fee was calculated “would tacitly suggest that all demands for payment in business transactions would need to be accompanied by an itemized list explaining the basis for each charge, so that the payor had full knowledge of the facts as required by the voluntary payment doctrine.” Putnam, 649 N.W.2d at 634. Even then, the payor could presumably challenge the basis asserted, with the result that the “full knowledge” requirement could never be met. It would be impractical and unfair to parties entering into late-fee agreements to interpret the “full knowledge” requirement in the way Peake proposes. We therefore decline to adopt an interpretation of the voluntary-payment rule that would eviscerate its purpose and create such an enormous burden on those who charge legitimate late fees in the regular course of their business.

We note that at least one court has held that customers who pay late fees with knowledge of the fee’s amount and the circumstances under which it will be charged, do so under a mistake of law rather than a mistake of fact. Putnam, 649 N.W.2d at 633. And it is well settled in Texas that payment made under a mistake of law is not an exception to the voluntary-payment rule. See Pennell, 243 S.W.2d at 575-76 (voluntary payment cannot be recovered “merely because the [payor] ... was ignorant of or mistook the law as to his liability”); Gilliam, 6 S.W. at 759 (“Money paid under a mistake of law ... cannot be recovered.”); Putnam, 649 N.W.2d at 635 (observing that a mistake of law “does not represent the type of wrongful action that should be excepted from the voluntary payment doctrine”); but see City of Taylor v. Hodges, 143 Tex. 441, 186 S.W.2d 61, 63 (1945) (refusing to apply the rule that “relief cannot be had from a payment made under a mistake of law” to situations in which “the payment is made with public money”). Thus, if Peake or any other club members paid BMG’s late fees under a mistake of law, i.e., believing that the fees were a reasonable estimation of damages and not an illegal penalty, they may still be charged with full knowledge of the facts for purposes of the voluntary-payment rule.

We recognize that the Indiana Supreme Court has taken a different approach to applying the voluntary-payment rule, one that it acknowledges is contrary to the great weight of authority. See Whiteman, 802 N.E.2d at 891. That court relied largely on the Tentative Draft of the Restatement (Third) of Restitution & Unjust Enrichment, which defines a voluntary payment as one “paid in the face of a recognized uncertainty as to the existence or extent of the payor’s obligation to the recipient.” Restatement (Third) of Restitution & Unjust Enrichment § 6 cmt. e (Tentative Draft No. 1, 2001). We agree with the Draft Restatement’s articulation of the full-knowledge requirement as contemplated by the voluntary-payment rule: