Q: A relative lent me money to publish a sefer. I repaid the loan. May I now give him a complimentary copy of the sefer as a token of appreciation?

A: The Torah prohibits only ribbis that was stipulated (ribbis ketzutza). However, the Sages prohibited - during the course of the loan or when repaying the loan - even ribbis that was not stipulated (ribbis she’aina ketzuza). Most poskim maintain that this is prohibited even if the additional payment is called a “gift” (Y.D. 160:4; Bris Yehuda 5:2).

Moreover, the Sages prohibited even ribbis given beforehand to secure the loan (ribbis mukdemes) or afterwards as compensation for the loan (ribbis me’ucheres).

Therefore, even after the loan is repaid, it is prohibited to give the lender a gift that is explicitly linked to the loan. However, it is permitted to give a modest gift after the loan is repaid, if not explicitly linked to the loan. A large gift should not be given until a significant time elapses (160:6; The Laws of Ribbis 2:23).

If you would have given the relative a complimentary copy even had he not loaned you, then it is permitted to give him a copy even before repaying the loan, provided that you do not link the gift to the loan (160:7).