What is the ruling on trading currencies and metals in international stock exchanges using the margin system, according to the mechanism mentioned, and does it become permissible upon owning the full value of the deal or upon paying part of it with the broker completing the remainder?

It is not permissible to enter into a deal by borrowing from a financial intermediary who takes a commission, because this is a loan that draws a benefit, and it is forbidden. The benefit is the amount of commission that the intermediary takes, and it is the same whether the intermediary finances part or all of the deal, and whether the client owns the full amount or not, as long as the intermediary will finance the deal.

However, if the intermediary's role is limited to providing market access, price indications, and financial analyses for a commission, and you pay the full value of the deal yourself, then this is permissible, provided that the conditions of currency exchange (sarf) are observed.

So, whenever the amount given by the intermediary to the client is guaranteed without participation in profit or loss, it is a loan, and every loan that draws a benefit is usury (riba).