Is dealing in foreign currencies when returning a trust considered usury if the exchange rate differs between the time the money was needed and the time of repayment?

If you lent money to your friend, he must repay the debt in the same currency he borrowed, or in an equivalent value in another currency at the exchange rate on the day of repayment. It is commendable for him to add an extra amount upon repayment as a sign of good fulfillment.

However, if you did not authorize him to borrow, he has committed a wrong and must repent and return the money in the same currency he took, or its value on the day of the transgression. There is no harm if your friend compensates you for any harm that befell you.