Is the additional amount paid after currency exchange considered usury (riba) or unlawful (haram)?

Exchange (Sarf) is only valid if there is immediate possession (taqabud) in the contract session. Otherwise, it falls under the usury of delay (riba al-nasiah), which is forbidden by scholarly consensus. If the exchange contract becomes invalid due to lack of immediate possession, both exchanged items must be returned. It is not a condition for the validity of the exchange that it be at market price; rather, it is valid at whatever price the two parties agree upon. If a specific price was agreed upon and immediate possession took place in the session, the other party has no right to demand an increase. If the entire amount is not taken possession of in the contract session, the exchange becomes invalid for the unpossessed portion, and this amount must be returned or another exchange performed for it.