Is it considered usury to take an unspecified additional amount upon settling a debt, if it is stipulated by the debtor as a reward?

If the borrower pays back more than he borrowed without a prior condition or agreement, there is no harm in that, and several hadiths support this. However, if there was a prior agreement on the increase, it is not permissible to pay or take it, because that is considered usury (riba). The majority of jurists hold that if the borrower pays back better than what he borrowed without a prior condition, then that is permissible.