We do not know a legal principle indicating the permissibility of what you say,
as (Part No. 14; Page No. 134) it is impermissible for a lender to take anything from the borrower except for the borrowed sum. However, if the borrower willingly gives the lender more than the borrowed amount, this is permissible as long as it is not preceded by a condition or a previous agreement to do so, even if it is customary, for there is no difference between a customary condition and a verbal one, as the Prophet (peace be upon him) stated:
The best of people are those who are the best in paying off their debts.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.