What is the ruling on taking a brokerage commission from both parties, or from one of them without the other's knowledge?

The broker's and intermediary's fee is permissible and is determined by agreement or custom. It is paid by the seller, the buyer, or both, provided that it remains within the customary limits to avoid harm.

If the broker works for one party, it is not permissible for him to collude with the other party to increase or decrease the price, as this involves deception and a breach of trust. If he profits anything, it belongs to his client.

However, if the broker's work is in exchange for a specific commission from one of the contracting parties, he is not obliged to inform the other party of this, unless the commission leads to an excessive unfairness in the price. Rather, the broker's fee is added to the price, as in a Murabaha sale. Cases where there is harm to the buyer or deception should be excluded.