Is it permissible to repay a general loan with something better in quality or quantity after repayment without prior condition?
Chapter on Loan (Qard)
Al-Mughni
Book of Sales
Primary text
If a loan is made absolutely, without stipulation, and the borrower repays with something better in quantity or quality, or less, with the mutual consent of both parties, it is permissible. This also applies if the repayment is made via a bill of exchange or in a different city. This view is supported by Ibn Umar, Sa'id ibn al-Musayyib, Al-Hasan, Al-Nakha'i, Al-Sha'bi, Al-Zuhri, Mak'hul, Qatadah, Malik, Al-Shafi'i, and Ishaq. The basis for this permissibility is the action of the Prophet (peace be upon him), who borrowed a young camel and returned a better one, stating, "The best among you are those best in fulfilling their obligations." Since the excess was not stipulated as a condition or substitute in the loan contract, it is lawful.
Supporting text
Some scholars, including Ubayy ibn Ka'b, Ibn Abbas, and Ibn Umar, maintain that one should only take the exact equivalent of the loan and not accept any excess, as accepting an excess constitutes a loan that draws a benefit. Another opinion suggests that if a borrower repays a loan with something better, and subsequently seeks another loan, the lender must only accept the exact equivalent of the second loan, otherwise it becomes forbidden Riba.