What is the religious ruling on not giving the broker the remaining 500 EGP of the commission, and instead giving it to the buyer, knowing that the broker has already received another commission from the buyer?

The questioner is obligated to fulfill the agreed-upon commission with the broker, due to the generality of Allah’s saying: "O you who have believed, fulfill [all] contracts," and the saying of the Prophet, peace and blessings be upon him: "Muslims are bound by their conditions." What the broker takes as an excess from the buyer does not require the knowledge or consent of the questioner, but rather the knowledge and consent of the one from whom it was taken. There is no harm for the broker in taking a commission from both parties without the knowledge of the other party.