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.