A friend used my credit card to pay for several flights over three months time. The card was charged in dollars but she is paying me in Israeli shekel. The first time she paid was before she traveled and she gave me an extra 100 shekel towards the return trip. Then she came back and gave me money for the return ticket though I am not sure how exact it was since I did not see my credit card statement until later and she probably rounded out the amount. Now she is paying me for the other 2 flights charged on my card. Every time
I check online to see how much the shekel equivalent is it changes and I certainly don’t know how much it was when the charge was actually processed. How do we resolve this so she pays me the right amount without ribis and without my losing anything.
The technical amount of money that she owes you will depend on what was agreed when she booked the flights. If it was agreed that the money would be paid in shekalim, then she owes you the shekel amount of the day that she booked the flight, because it was from that day that she owed you shekalim. You can find out the shar yatzig of any day in the past at the bank of Israel site. You can use this link https://www.boi.org.il/he/Mobile/Pages/ExchangeRates.aspx However if it wasn’t clear that it would be paid in shekalim then the amount owed would be according to the rate of the day that the money was paid back. This is the technical amount that she owes you.
Practically though, the fact that she doesn’t know exactly how much to pay you, does not mean that you have to take a loss, she can give you a little bit extra, to make sure that she surely paid you what she owe you and it will not be ribis. This is because she is not giving the extra money to you because of an extension of the loan, but for her own benefit, to make sure that she is yotza yidei shomyim, and you are allowed to accept it.
Bris PInchos 16-16, 5-19 quoting the Raavad (Tomim Deim) Siman 60.