What are the conditions for legitimate currency trading, and is the amount taken by the broker for keeping the position open until the next day considered usury (riba)?

It is permissible to trade currencies, subject to Sharia compliance. If the buyer pays part of the amount and the broker completes the remainder, and the sale and قبض (taking possession) occur in the same session of the contract, then it is permissible. This is because the broker acts as an agent for the buyer, and it is permissible for him to lend to the buyer. The broker's fee must be known, and it is not permissible to take an amount in exchange for the merchant holding the currency until the next day, as this constitutes a usurious increase on a debt.