I live in England and lent a sum of shekolim to a Yid who was visiting Eretz Yisroel for a short while. We agreed that he would pay me back in pounds for all the shekolim that he used and the rest he would return unused. The exchange rate that we would use would be whatever it is at the time that we calculate. He is now back in England and we must get round some time to make a cheshbon. Could you clarify for me that if this arrangement is ok, for the perspective of Hilchos Ribbis.
Before discussing the shekalim that he used, there is an issue with the shekalim that he didn’t use. This is because he took them at one rater and he returned them when they were at a different rate, which would be “seah b’seah”. (The shekalim are considered “peiros” in England, and therefore since the rate changed you might have given him the peiros when they were cheaper and got them back when they were more expensive.) Therefore you should check up what the rate was on the day that you gave it and received it, and if the rate on the date that you received it was. If the shekalim increased in value then “lotzeis y’day shomayim” you should return the difference to him. ( If the value of the shekel went down then you got back less value from him and it is fine.) If however the person who borrowed the money owned some shekalim at the time that he borrowed them, then what was done is fine.
Regarding the shekalim that he did use. If you would have said that it will be according to the rate of the day that he pays you back, then it would be an issue, because you might be getting back more value then what was given. However since you made up with him that it will be according to whatever date the both of you decide to calculate then it is alright to calculate according to that day’s exchange rate. There is however a different issue. If the term used when giving the money wad that it is a loan, then even though you made up that he would pay the money back at the rate of the specific date, it would still be considered, like you lent him the money at a lower price and took it back at a higher one. Therefore if the term lend was used, and not sell, then you will have to use the rate of the day that the money was given, or if the rate on the day it is paid back is less, then it can be returned at that rate.
Shulchan Aruch Y:D 162-1,2, 161-2, Avnei Nezer Y:D 207-4, Bris Pinchos 16-16.