Ruling on Repaying a Loan When the Value of the Currency Changes

Question: I inform you that I borrowed a sum of money from a person who does not follow Islam; this was due to dire necessity, on the condition that I would return to him the equivalent value of the amount in a free currency; that is, a currency other than my country's currency, and that I would do so when I returned to my place of work in Saudi Arabia. When I returned after a period of time, the value of the free currency had risen and became equal to twice the amount I had borrowed. So, is it permissible to send him the amount in the free currency—despite the currency difference—or should I send him only the amount I borrowed?

Answer: This loan is invalid; because in reality it is a sale of a present currency for another currency on credit, and this is a transaction involving usury or interest; because it is not permissible to sell a present currency for another currency except at the rate of the day, hand to hand. And you must return to him only what you borrowed from him, along with sincere repentance for the usurious transaction that took place.