Al-Fath ar-Rabbani 6016 — Book of Salam, Loans, and Debts

It is not permissible to give a loan with the intention that something better will be received at the time of repayment; however, the debtor may, on his own accord, repay in a better manner. In any case, it is not permissible for the lender to have such greed. (This passage is from the commentary, which is why the numbering has become incorrect; since 6025 is missing, the numbering becomes correct after that.)