Is it permissible, Islamically, for someone who borrowed a sum of money and promised to repay it after harvesting his crop with a specific quantity of rice, the value of which is estimated at the borrowed amount, to pay the lender the price difference if the value of the rice increases at the time of repayment, knowing that the prevailing custom in that region is to repay debts from the crop regardless of subsequent price changes?

The Prophet, peace and blessings be upon him, approved the transaction of the Ansar in which an advance payment was made for the future purchase of dates. This transaction is known as 'Salam' or 'Salaf'. It is permissible for a person to take the agreed-upon rice even if its price increases, because 'Salam' is the prepayment of the price and the deferment of the sold item under known conditions (a known measure, a known weight, and a known term). This is not a loan, but rather a sale.