Is it permissible to pay back a debt, borrowed in Egyptian Pounds, in US Dollars, at the exchange rate prevailing at the time of borrowing, not repayment, with the intention of showing gratitude to the lender?

It is not permissible for the lender and borrower to agree, before the loan's due date, to repay it in a different currency, as this involves riba al-nasi'ah (usury of delay), for currency exchange is only valid with immediate قبض (seizure/possession). It is permissible to agree on the due date to repay the debt in a different currency at the exchange rate of that day, provided that no part of the currency exchange remains as a liability. It is not permissible to register the debt as equivalent to the value of gold or another currency, with the debtor being obliged to repay the debt in them. As for rewarding the lender after repaying the loan, it is recommended, by adding money as a form of recompense for kindness, based on the saying of the Prophet, peace and blessings be upon him: "Indeed, the best of people are those who are best in settling their debts."