Is it permissible for a woman to return to her husband the properties (real estate and investment certificates) that he had gifted to her, after her death, by writing a will to that effect?

It is not permissible for you to bequeath anything from your estate to your husband, because he is one of your heirs, and bequeathing to an heir is legally forbidden, based on the Prophet, peace and blessings be upon him, who said: "Indeed, Allah has given every rightful person their due right, so there is no bequest for an heir." This bequest, however, becomes valid and depends on the approval of the heirs.