P.R. Laws tit. 31, § 5121 : ONE Civil Code —Subtitle 4 Obligations and Contracts —PART XVI. Obligations Contracted Without Agreement —Chapter 391. Quasi Contracts —Subchapter II. Collection of What is Not Due —§ 5121. Restitution of thing improperly received
If a thing is received when there was no right to claim it and which, through an error, has been unduly delivered, there arises an obligation to restore the same.