Is it permissible to buy a house that the deceased bequeathed to one of his sons while disinheriting the rest?

What this person did is null and forbidden for two reasons: First, he made a bequest to an heir, and a bequest to an heir is not valid unless the other heirs agree, as the Prophet, peace and blessings be upon him, said: "Indeed, Allah has given every rightful person his due, so there is no bequest for an heir." Second, he did not treat his children justly in giving gifts, and the Prophet, peace and blessings be upon him, called that injustice.

Therefore, the legatee must return the bequest to the estate so that it can be divided among the remaining heirs according to what Allah has legislated, because the bequest is null and void, and what is null and void is not enforceable, as the Prophet, peace and blessings be upon him, said: "Whoever performs an action which is not in accordance with our matter (religion) will have it rejected."

The condition for the validity of a sale is that the commodity must be owned by the seller. The ownership established by the aforementioned bequest is not lawful, so it is not permissible to sell, buy, or gift it because it is based on aggression, and the legatee is usurping the rights of the other heirs. This is based on the saying of the Prophet, peace and blessings be upon him: "There is no right for an oppressive root."

The actions of the usurper are null and void, and they are considered as non-existent. It is forbidden to buy stolen or usurped goods, or goods not owned by their seller, because that involves cooperation in sin and aggression, depriving the true owner of their goods, and condoning evil.