How can one engage in forex trading in a halal manner, taking into consideration that leverage (margin) is not considered an interest-based loan, but rather closer to a pledge or agency?

The financial leverage or margin provided by the broker to the client is a loan. The fact that the account holder cannot withdraw the amount does not negate its nature as a loan, and it is usury (riba) because every loan that draws a benefit is usury. The jurists have agreed that every loan that brings a benefit is forbidden usury. Furthermore, the broker's condition that trading must be conducted through him leads to combining a loan with a compensatory transaction, which is akin to combining a loan with a sale, and this has been forbidden by Islamic law.