Is borrowing an amount to buy from a brokerage firm considered "a loan that draws a benefit" if the company profits from the difference in buying and selling prices as a commission?

Brokerage firms do not lend to clients except on the condition that their transactions for the loan amount are exclusively through them. This leads to the combination of a loan (salaf) and an exchange (mu'awadah), which is tantamount to combining a loan and a sale, which is prohibited by Islamic law. Thus, every loan that draws a benefit is usury (riba) and forbidden.