If someone asks for 10 grams of gold or its actual market value as a loan, and returns it back with either gold or its actual market value, then there is nothing wrong with this process. But, it is very important to make the exchange according to a rate that is not defined by the borrower or the lender. The exchange rate should be defined by a 3rd party, like for example the central bank. The parties cannot agree on a future exchange rate while giving/receiving the loan.
For short, the jeweler (lender) shouldn’t make a profit out of this process. Making profit out of a loan is interest.
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Assoc. Prof. Servet Bayındır