Muwatta Malik #1438

Malik related to me from Humayd ibn Qays al-Makki that Mujahid said, "Abdullah ibn Umar borrowed some dirhams from a man, then he discharged his debt with dirhams better than them. The man said, 'Abu Abdar-Rahman. These are better than the dirhams which I lent you.' Abdullah ibn Umar said, 'I know that. But I am happy with myself about that.' " Malik said, "There is no harm in a person who has borrowed gold, silver, food, or animals, taking to the person who lent it, something better than what he lent, when that is not a stipulation between them nor a custom. If that is by a stipulation or promise or custom, then it is disapproved, and there is no good in it." He said, "That is because the Messenger of Allah, may Allah bless him and grant him peace, discharged his debt with a good camel in its seventh year in place of a young camel which he borrowed, and Abdullah ibn Umar borrowed some dirhams, and repaid them with better ones. If that is from the goodness of the borrower, and it is not by a stipulation, promise, or custom, it is halal and there is no harm in it."