Is it permissible to increase the repayment of a loan if the increase comes from the borrower himself and was not a stipulated condition?

If your brother's request for a loan explicitly stated that he would repay you more than you lent him, and you gave him the money on that basis, then this is plain and forbidden usury (riba). However, if he did not mention any increase, and you gave him the money on the basis that it was a loan to be repaid in kind, and then he gave you more upon repayment, this matter is subject to scholarly disagreement. The majority of scholars deem it permissible, and this is the preponderant view.