Returning the Ribis that one’s parent collected from a loan:
A child is to return to its rightful owner an item that was taken by his father or mother from a Jewish borrower as interest for a loan that the father gave. This, however, only applies if the item is potentially recognizable to others that it belonged to someone else and was stolen by his parent, such as a piece of furniture, blue, or clothing. However, money, as well as other items which are not recognizable as having belonged to someone else, do not have to be returned. Likewise, the above obligation only applies if the parent has repented his act of taking interest from a Jew but simply did not get a chance to return the interest item.
 Michaber Y.D. 161:6; Rambam Malveh Veloveh 4:4; Bava Kama 111b; Kesef Mishneh Mamrim 6:11; Encyclopedia Talmudit ibid p. 461 footnote 1205