What is the ruling on giving a sum in dollars to a merchant so that his agent may transfer it in dollars to another agent in another country, given that the transfer is not immediate and the merchant's fee is a percentage of the amount?

If a merchant takes a sum of money from a sender and then gives another sum to the sender's agent in another country, this is considered an exchange (mu'awadah) and a currency exchange (sarf), and it requires immediate possession (taqabud) and equality (tamathul). If immediate possession does not occur, meaning the sum is delivered at a later time, then it falls into Riba al-Nasi'ah (interest due to delay). If a smaller sum is delivered after deducting his commission, then the condition of equality is violated, and it falls into Riba al-Fadl (interest due to excess). Therefore, this form of transaction is not permissible.