What is the ruling on selling one currency for another with a profit resulting from the market rate, and what is the ruling on profit resulting from simultaneous multi-currency trading?

"Trading currencies is permissible, provided that the exchange takes place during the contract session, such as selling Riyals for Euros and then converting the Euros into Dollars. The proof for this is the saying of the Prophet, peace and blessings be upon him: 'Gold for gold... If these types differ, then sell as you wish, provided it is hand to hand,' as stated in the fatwas of the Permanent Committee."