What is the ruling on the action of a real estate broker who oversteps the original agreement with another broker to obtain the full sales commission from the property owner?

The brokerage contract is one of the permissible contracts that can be rescinded. The property owner has the right to rescind the contract and conclude it with another person. If the broker begins work, he is entitled to a customary wage (ajr al-mithl) for his effort. The broker who broke the covenant must repent to Allah Almighty, because Allah Almighty says: "O you who have believed, fulfill [all] contracts."