Is exchanging money for money, the amount of which is only estimated, forbidden in Islamic law or permissible? If it is not permissible, what should be done?

If you give your friend money of the same type as his coin, it is not permissible. This is due to the non-fulfillment of the two conditions of immediate exchange and equality in the same session. However, if the currencies differ, then disparity is permissible, provided immediate exchange is stipulated. Paper and metal currencies stand in the place of gold and silver, and they are categories that multiply with the multiplicity of issuing authorities. Since the amount he gave you is unknown, the sale is void. Therefore, pay your friend his coin, and let him return what you paid him. If you both wish to make a transaction, count the coins, and give him an equal amount in paper currency, as long as the currency is the same.