Is it considered a sin for an engineer to take a commission on raw materials and labor without informing the apartment owner, or is it permissible as a form of trade, knowing that the apartment owner approved the costs presented at each stage of the finishing work?

It is not permissible for the engineer to take money for himself from the apartment's finishing without the owner's knowledge or agreement, because the Prophet, peace and blessings be upon him, said: "The two parties to a transaction have the choice as long as they have not parted. If they are truthful and open, their transaction will be blessed. But if they lie and conceal, the blessing of their transaction will be erased." And because he said: "It is not permissible to take the property of a person except with his willing consent." If he does not have a known wage and is not volunteering, then he is entitled to a fair wage.