What is the ruling on taking a commission for exchanging foreign currencies if the funds of both parties are permissible? And what is the ruling on taking a commission from the buyer with his consent and knowledge, without the seller's knowledge, if the commission is only from the buyer's funds and the seller has received the price he desired?

If the transaction is conducted in accordance with Sharia regulations, especially the immediate exchange (qabd) at the contract مجلس (gathering) when two different currencies are involved, then there is no harm in brokerage (samsarah) and taking a commission for it from one or both parties, provided that the party from whom the commission is taken is aware of it.