What is the ruling on a person buying banknotes from another at a specific price, then selling them at a higher price and profiting from the price difference, knowing that he will return the agreed-upon amount to the first person after selling the banknotes?

Exchanging one currency for another is called sarf (currency exchange), and it is only valid if both عوض (counter-values) are taken possession of in the same contractual session. Otherwise, it constitutes riba al-nasia (usury of delay). Scholars have unanimously agreed on the invalidity of currency exchange if the two parties separate before taking possession of the exchanged items. Therefore, if one currency is exchanged for the same type of currency or for a different one, mutual possession must take place in the contractual session; otherwise, the contract is null and void. Accordingly, it is not permissible to purchase currency from your friend without handing over its equivalent in the same contractual session.