What is the ruling of Islamic law regarding a vow to bestow a plot of land upon a brother, knowing that the brother is an heir and that the vow-maker's financial circumstances have changed, and he has become indebted? Is this vow considered valid, or is it void?

If a brother bequeaths land to his brother after his death, then the will is a non-binding contract, and the testator is permitted to revoke his will, even without cause, because it is a gift or a donation that is not completed except by death, and because acceptance is contingent upon death. The testator has the right to invalidate the offer before acceptance. Therefore, this brother has the right to revoke and invalidate his will, and his protesting brother has no right in this matter.