PROMUTUUM, civil law. A quasi contract, by which he who receives a certain sum of money, or a certain quantity of fungible things, which have been paid to him through mistake, contracts towards the payer the obligation of returning him as much. Poth. De l'Usure, 3eme part. s. 1, a. 1.
2. This contract is called promutuum, because it has much resemblance to that of mutuum. (q. v.) This resemblance consists, 1st. That in both a sum of money or some fungible things are required. 2d. That in both there must be a transfer of the property in the thing. 3d. That in both there must be returned the same amount or quantity of the thing received. Poth. h. t., n. 133. But though there is this general resemblance between the two, the mutuum differs essentially from the promutuum. The former is the actual contract of the parties, made expressly, but the 'latter is a quasi contract, which is the effect of an error or mistake. Id. 134; l Bouv. Inst. n. 1125-6.