Is it permissible for me to stop paying 5% to a person who introduced me to a client, knowing that the client is unaware of this agreement, and that the person refuses to cancel the agreement and threatens not to forgive?

This transaction is not permissible according to the majority of scholars because if the broker's commission is a percentage and not a fixed amount, the uncertainty of the compensation (ju'l) prevents its validity. The Hanbalis, however, permit it if the uncertainty does not lead to dispute. As for a commission after the first sale, the broker is not entitled to it due to the absence of any work on his part. As stated in the Fiqh Encyclopedia, profit is only deserved through capital, work, or guarantee. Therefore, if the broker stipulates a commission for something he did not work on, the condition is void.