Question: My friend purchased something on my behalf, and as a result I owe him $19.50. Am I allowed to pay him $20 (and the extra 50 cents will be a gift) or is this a problem of ribbis?
Answer: This is a problem of ribbis because the discrepancy – 50 cents – although it is a paltry sum, it does have significance, therefore you are not allowed to return more than is owed, even if you state that the extra 50 cents is a gift (Shulchan Aruch Yoreh De’ah Siman 160 Se’if 5 “…even if he [the borrower] says to him [the lender] I am giving the extra to you as a gift – it is forbidden…”, Shach ibid s.k. 6 “and this even pertains to ribbis d’rabonon”, Toras Ribbis Perek 3 Se’if 8).
[The reason that this is ribbis d’rabonon and not ribbis min hatorah is because it was not stipulated at the time of the loan that the borrower will pay interest.]