Is it permissible to deal with a brokerage company that allows margin trading in currencies, knowing that it does not charge interest for covering the transaction (the margin), and if it is impermissible, is it permissible to continue with it to recover the capital?

The forbidden aspect in brokerage firms is lending the client a multiplied balance against their investment of funds through the firm, because "every loan that draws a benefit stipulated in the loan contract is usury (riba)." However, if the investor deposits their money with the firm for speculation in buying and selling currencies, without stipulating a loan to them, or lending them without stip stipulating dealing through the firm, and the firm executes their buy and sell orders for a defined commission, then there is no objection to dealing with it, provided that the conditions for the permissibility of currency exchange, such as immediate possession (taqabud), are met.