Answer
Solutio indebiti is a quasi-contract that arises when a person receives something that was not due to him and it was unduly delivered to him through mistake; the law then obliges him to return it (Civil Code Article 2154). Its requisites are that there was a payment or delivery, that the person who received it had no right to demand it, and that the delivery was made through mistake — not through liberality or some other valid cause.
The obligation rests on the principle that no one may unjustly enrich himself at the expense of another (Article 22). If the payee acted in bad faith — knowing the thing was not due — he is also liable for the interest and for the fruits received or that he should have received.
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