Q. Someone borrowed a sefer, but kept it for a long time, then graciously to thank for the favor returns it with a bottle of wine as an act of thanks. Is that a question of ribbis or prohibited interest?
A. Poskim rule that if the borrowed item can decrease in value by being used (yesh lo p’chas), it is permitted to give special thanks or a gift when the item is returned. (Parshas Ribbis 12: 17, and others).
Horav Shlomo Miller’s opinion is similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a