What is the ruling on taking the difference in price after agreeing upon a certain price, and is it permissible to take a percentage from the seller or the buyer in return for bringing a customer?

The commission (brokerage fee) must be based on a contract of a known amount between the two parties to the contract. It is stipulated that the party paying the commission must be aware of it. If it is taken from the seller alone, then his knowledge is required, not the buyer's. If it is taken from the buyer alone, then his knowledge is required, not the seller's. It is not permissible for the broker to take the price difference for himself without the buyer's knowledge if he finds the merchandise at a lower price than agreed upon, as opposed to if he agreed with the seller on a specific commission in exchange for bringing a client.