Undue payment (solutio indebiti)

One of the most important categories of unjust enrichment, solutio indebiti, was the mistaken payment of a debt in discharge of an obligation that in fact did not exist. The legal act, not founded on contract (Gaius 3.91), but con­sidered a quasi contract, was almost identical to the mutuum, except that the payment was made in discharge and not to create an obligation.

Both the giver and the receiver had to act in good faith and in error. If the giver paid knowing that the money was not due, the payment was not

recoverable (Ulpian, D. 12.6.1) and therefore was treated as a donation. If the receiver acted in bad faith, the payment was treated as a theft. Payment of a debt of money due by a third person was an undue payment unless paid in the name of the third person. Paul says, however, that no recovery is possible against a man who has received his own property (a slave, for instance), even from a person other than the true debtor (D. 12.6.44). A conditional debt remained undue as long as the condition was not satisfied (Pomponius, D. 12.6.16pr.). However, where a debt fell due on an unfixed day (dies incertus), recovery was not possible because the day had to come first (Pomponius, D. 12.6.16pr.). The fulfillment of a natural obligation could not be considered undue payment (Africanus, D. 12.6.38.1).

The giver could recover the payment by a personal action: the condictio indebiti (Gaius 3.91). The plaintiff had to prove the fact of the payment and that the obligation was not due. Restitution had to be with fruits and accessories, and interest could not be claimed.

Pacta

A pactum (plural, pacta) was “the agreement and consent of two or more persons concerning the same matter” (Ulpian, D. 2.14.1.2). All kinds of agreements that did not fall within a defined category of contract could be considered a pactum. Originally, pacts were not actionable, and they gave rise only to a defense (Ulpian, D.

At times, however, pacta were enforceable and gave rise to an action (Ulpian, D. 2.14.7.5). Enforceable pacts were the so-called pacta adiecta, i.e., an additional agreement to a contract in order to modify part of its content - e.g., the agreement by means of which a tenant would be liable for whatever occurred due to a higher force (vis maior) (Ulpian, D. 19.2.9.2) and the aforementioned lex commissoria in the sale, among others. If the additional agreement was made at the time of the original contract, the agreement was enforceable by the very contractual action. When the agreement was intro­duced subsequently, it was not enforceable by means of an action, but only an exception.

Sometimes, the praetor granted an action in factum for the enforcement of some agreements that were not additional to contracts. That occurred, for instance, in the case of an informal agreement to pay an already existing debt at an agreed-upon, fixed date (constitutum debiti). The debt could be one’s own or that of a third person, so it constituted a form of surety. The agreement had no novatory effect because it was not a stipulation. The advantage of bringing the action in factum (called actio de pecunia constituta) was that it

The law of obligations: contracts 203 was strengthened by an additional promise of a penalty of one half of the original debt.

Other praetorian agreements were the undertaking of a person to act as an arbitrator in a dispute between two or more persons (receptum arbitri); the obligation assumed by a banker to pay the debt of his client at a fixed date (receptum argentarti); and the undertaking of a ship’s master, an innkeeper, or a liveryman that their customers’ goods confided to them would be safe (receptum nautarum, cauponum, stabulariorum).

Further reading

Berger, Adolf. Encyclopedic Dictionary of Roman Law. Philadelphia: The American Philosophical Society, 1953; reprint 1980.

Betti, Emilio. La struttura dell'obbligazione romana e il problema della sua genesi. Milan: Giuffrè, 1955.

Birks, Peter. The Roman Law of Obligations. Edited by Eric Descheemaeker. Oxford: Oxford University Press, 2014.

Buckland, William Warwick. A Text-Book of Roman Law. 3rd rev. ed. Revised by Peter Stein, 405-603. Cambridge: Cambridge University Press, 1963.

Diosdi, Gyorgy. Contract in Roman Law: From the Twelve Tables to the Glossators.

Translated by J. Szabo. Budapest: Akademiai Kiado, 1981.

d’Ors, Àlvaro. Derecho privado romano. Edited by Xavier d’Ors. 10th ed. Pamplona: Eunsa, 2004.

du Plessis, Paul J. Letting and Hiring in Roman Legal Thought: 27 bce-284 ce. Leiden: Brill, 2012.

Fiori, Roberto. La definizione della locatio conductio: giurisprudenza romana e tradizione romanistica. Naples: Jovene, 1999.

Grosso, Giuseppe. Il sistema romano dei contratti. 3rd ed. Torino: G. Giappichelli, 1963. Ibbetson, David J. A Historical Introduction to the Law of Obligations. Oxford and

New York: Oxford University Press, 1999.

Ibbetson, David J. “Obligatio in Roman law and Society.” In The Oxford Handbook of Roman Law and Society, edited by Paul J. du Plessis, Clifford Ando, and Kaius Tuori, 569-580. Oxford: Oxford University Press, 2016.

Kaser, Max. Das romische Privatrecht. Vol I, Das altromische, das vorklassische und klassische Recht, 474-667. 2nd ed. Munich: Beck Verlag, 1971.

McGinn, Thomas A. J., ed. Obligations in Roman Law: Past, Present, and Future. Ann Arbor: University of Michigan Press, 2012.

Mousourakis, George. Fundamentals of Roman Private Law, 183-251. Berlin and Heidelberg: Springer, 2012.

Nicholas, Barry. An Introduction to Roman Law, 158-207. Oxford: Clarendon Press, 1975. Paricio Serrano, Javier, ed. Derecho romano de obligaciones. Homenaje al profesor Jose

Luis Murga Gener. Madrid: Centro de Estudios Ramon Areces, 1994. Santoro, Raimondo. Il contratto nel pensiero di Labeone. Palermo: Palumbo, 1983. Schulz, Fritz. Classical Roman Law, 455-633. Oxford: Clarendon Press, 1951. Thomas, J. A. C. Textbook of Roman Law, 312-383. Amsterdam, New York, and

Oxford: North-Holland Publishing Company, 1976.

Watson, Alan. Contract of Mandate in Roman Law. Oxford: Oxford University Press, 1961.

Watson, Alan. The Law of Obligations in the Later Roman Republic. Oxford: Clarendon Press, 1965.

Wegmann Stockebrand, Adolfo. Obligatio re contracta. Ein Beitrag zur sogenannten Kategorie der Realverträge im romischen Recht. Heidelberg: Mohr Siebeck, 2017.

Zimmermann, Reinhard. The Law of Obligations. Roman Foundation of the Civilian Tradition. Oxford and New York: Oxford University Press, 1996.

Zulueta, Francis de. The Institutes of Gaius. Part II. Commentary. Oxford: Clarendon Press, 1953.

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