Does usury occur upon قبض (taking possession) if the exchange value paid differs from what was previously agreed upon, with the intention of paying the difference later, knowing that the intention was solely on the part of the buyer?

The exchange of different currencies requires immediate possession in the same session, as they are subject to the rulings of gold and silver. The Islamic Fiqh Academy has stipulated that paper currencies possess full monetary value and are subject to the rulings of Riba (usury) and Zakat (alms). Furthermore, deferred payment sales of currencies are not permissible according to Sharia. If the seller agrees to a lower price and immediate possession occurs, the buyer is not obligated to pay him the difference. If he gives him the difference later, it is a permissible gift if the giver is legally competent. However, if he gives him the lesser amount with an agreement or promise to give the difference later, this is a forbidden currency exchange (sarf) due to the delay in possession. If it was only an intention without an agreement, then the exchange is valid, and what is paid later is considered a gift.