Is the brother entitled to claim his father's inheritance in the family apartment, after he had previously waived his right to it in writing in exchange for receiving another apartment from his father before his death?

This is a judicial matter that requires a ruler's decree to resolve the dispute, with the caveat that a son's relinquishment of his share of his father's inheritance while the father is still alive has no legal validity (in Islamic law); because the son might die before his father, thereby becoming an inherited person (موروثًا) rather than an heir (وارثًا). What occurred is merely a promise to relinquish. For a right is not forfeited until it becomes due and the cause for its being due exists. Anything before that is not considered a forfeiture but merely a promise.