I am afraid that I have only recently started to study this kind of thing and I completely unlearned compared to most of the contributors here, so I apologize if my question is somewhat unsophisticated.
Consider a Jewish person who grew up in the UK, but lives in the US. Suppose that an old friend of his (who is Jewish), living in the UK, asks him for a loan. If everyone still lived in the UK it would be easy: the loan would be for a fixed number of British pounds, with no interest. And if everyone lived in the US, it would be easy: a fixed number of US dollars, with no interest. But here the lender and borrower live in different countries, with different currencies. If the loan is made in dollars, and the dollar appreciates, then the borrower will end up paying more back (in pounds, the currency they use every day) than they borrowed, which looks like ribis. But if the loan is made in pounds, and the dollar depreciates, then the lender will end up receiving back more in dollars (the currency they use every day) than they lent, which also looks like ribis.
Is there some simple rule like 'work in the lender's currency'? Or something more complicated? Or is such a transaction such as this simply impossible? (The latter would be sad, given the positive commandment to lend money to those in need of it.)
I know that the relevant standard is meant to be monetary value, but I don't see how to apply that here since there are two relevant notions of money.
UPDATE A quick point of clarification: it's hard to give a good clear answer to the question 'in which country would the loan be made'. The lender lives in the US, and would probably just instruct his bank in the US to do a bank transfer to the UK. The borrower lives in the UK, and would just receive the bank transfer. Returning the money would work in a similar way. So each person would always just be staying in their own country.