Is it permissible to trade in hard currency?

The Fiqh Academy has affirmed that paper currencies are considered money with full monetary value, and they are subject to the rulings of gold and silver regarding Riba (usury), Zakat (almsgiving), and Salam (forward selling). Therefore, Riba applies to currency exchange; it is prohibited to exchange different denominations of the same currency with a disparity in amount or with delay. However, when exchanging different currencies (e.g., the currency of one country for that of another), delay is prohibited, but disparity in amount is permissible, provided that the exchange takes place hand-to-hand in the same session. Trading in currencies is not permissible if the state prohibits it for a legitimate interest.