What is the ruling on a company deducting a portion of the commission of brokerage firms as a marketing fee without increasing the cost to the client, and what is the ruling on converting currency for a client in the black market for a fee in exchange for bearing the risks of conversion without informing them of the benefit from the commission?

The first issue is not clear. It is permissible to exchange currency on the black market if the conditions for exchange are met. It is not permissible for an agent to take a profit without the knowledge of his principal, because the profit belongs to the principal, unless the principal gives permission. It is not permissible for a Muslim to expose himself to harm.