What is the ruling on one who borrows a sum of money, and the lender stipulates that it be returned in gold upon repayment after three years, knowing that the price of gold fluctuates?

What is called a loan here is, in fact, an invalid exchange (sarf fasid); because it is the sale of cash for cash, and it requires the mutual taking possession of the two عوض (countervalues) in the same session. This is based on the statement of Ibn al-Mundhir: "Everyone whose knowledge we preserve has agreed that if two parties exchanging currency (al-mutasarifayn) separate before taking possession of the two عوض (countervalues), then the exchange (sarf) is invalid." The current obligation is now to return the right to its owner from its own kind, not from gold.