What is the ruling on borrowing one currency and returning it in another, and is it permissible to agree to convert the borrowed amount into another currency to buy things for the lender?

It is permissible for the debtor to convert the amount he borrowed to another currency at the request of the creditor, so that the latter may purchase goods for him with it. This is because the creditor has authorized the debtor to spend the specific amount of his debt in another currency along with purchasing the goods, and agency is permissible in Islam.