Is the agreement signed between the heirs, which stipulates that one of the two apartments be owned by the brother who undertook their construction in exchange for his relinquishment of the other one he also built, Islamically valid, or should the inheritance be re-divided so that the brother owns both apartments in addition to his share of the remaining inheritance?

If your brother and the rest of the heirs agree to a settlement regarding one of the apartments, and everyone accepts it, then this settlement is valid and binding. No party is permitted to revoke it, for the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions, except for a condition that makes a forbidden thing lawful or a lawful thing forbidden." A settlement, if concluded according to its conditions, is binding. It should be noted that a definitive ruling on such multifaceted issues is obtained by referring to the Sharia court.