What is the ruling on an agreement between a debtor and a creditor that the debtor return the amount he borrowed to the creditor, in addition to another amount by way of charity or gift and not as interest? And what should he do if that is impermissible?

Naming the increase on a debt as a gift or charity does not change the fact that it is a usurious increase if the agreement in the loan contract includes it. It is permissible for the debtor to honor the creditor with an increase upon repayment without a prior agreement. Scholars have differed on the ruling of a debtor repaying more in a loan upon settlement without a condition, promise, or custom. Ashhab permitted it, while Ibn al-Qasim disliked it. The Prophet, peace and blessings be upon him, gave a better age of a camel in repayment of a loan and said: "The best of you are those who are best in fulfilling their obligations," provided there is no condition, promise, or custom.