Malik related to me from Humayd ibn Qays al-Makki that Mujahidsaid, "Abdullah ibn Umar borrowed some dirhams from a man, then hedischarged his debt with dirhams better than them. The man said, 'AbuAbdar-Rahman. These are better than the dirhams which I lent you.'Abdullah ibn Umar said, 'I know that. But I am happy with myself aboutthat.' "Malik said, "There is no harm in a person who hasborrowed gold, silver, food, or animals, taking to the person who lentit, something better than what he lent, when that is not a stipulationbetween them nor a custom. If that is by a stipulation or promise orcustom, then it is disapproved, and there is no good in it."He said, "That is because the Messenger of Allah, may Allah bless himand grant him peace, discharged his debt with a good camel in itsseventh year in place of a young camel which he borrowed, and Abdullahibn Umar borrowed some dirhams, and repaid them with better ones. Ifthat is from the goodness of the borrower, and it is not by astipulation, promise, or custom, it is halal and there is no harm init."
USC-MSA web (English) reference: Book 31, Hadith 91
Arabic reference: Book 31, Hadith 1377