Are transactions involving the buying and selling of currencies via phone on the stock exchange permissible, with profit or loss calculated, if the exchange acts as an agent for the money changer?

This transaction is permissible if it adheres to the Shariah guidelines, which are: that the amount deposited with the stockbrokers covers what they are asked to purchase for him of another currency, whether he covers it himself, or through a benevolent loan they grant him without stipulating any benefit or penalty clause for delay. And that the purchased amount is received and its equivalent is delivered immediately, whether hand-to-hand or by depositing both amounts into the account of the recipient. And that the sale of what they bought for him follows the same method of delivery and receipt by which they purchased it.