Is it permissible for the wife to gift her husband her share of the house they own jointly, or part of it, after her death, with the remainder of her estate being divided according to Islamic law?

A wife's gift of her share of the house to her husband after her death is not valid because a gift contingent upon death is considered a bequest (waṣiyyah), and a bequest to an heir is not valid unless all the heirs approve it after the death of the testator. This is based on the Prophet's (peace be upon him) saying: "There is no bequest for an heir."