l was on a trip to Europe, and I lent some money for my friend by paying some stuff for him with my debit card. (Which is an American card so it works with dollars.) (I expected him to pay me back in shekels, since we both learn in Israel, but we didn’t make up anything.)
When he paid me back a few weeks later in Israel, the dollar went up so a dollar is now worth more shekels than at the time I lent it.
So, does he have to pay according to the current exchange rate, or according to the exchange rate when I lent him the money?
If your friend had dollars at the time of the purchase, which were accessible to be sent to him etc., then he can pay you according to the current (higher) exchange rate. The reason I as follows. When you let him use your American credit card, you lent him dollars. Since you were in Europe at the time, it is considered like you lent him merchandise, (called saah b’saah). Therefore, he owed you the dollar amount of the purchase. Under the assumption that an American teenager owns dollars, then there was no issue with that type of loan since it is called “yesh lo”, he had the same type of merchandise in his possession. Since you were allowed to lend him the merchandise, when he decides to pay you back the dollar value of the purchase, he may do so even though the price of the dollar went up. Therefore, he may pay you at the dollar- shekel rate on the day the loan was returned.
Yora Deah 162-1, 2.