Is it permissible to benefit from half of the profit that the brother who borrowed the money promised?

It is impermissible to stipulate a benefit for the lender or to collude upon it, for a loan is among the contracts of beneficence, and every loan that draws a benefit is usury (riba). If you lend money to someone and he promises to give you something, do not accept it unless you count it as part of your debt. Scholars have explicitly forbidden the lender from accepting a gift from the borrower if it is given on account of the loan and was not customary before it, whether this is before repayment or during it, unless the gift is accounted for as part of the debt. However, if the borrower repays all that is due from him and then gives a gift without a prior agreement, there is no harm in accepting the gift.