Is it permissible for the heirs to convert the remaining one-third of their father's estate, bequeathed for charitable deeds, into an endowment (waqf), and who would be the supervisor (nazir) of this endowment?

A will is permissible by agreement of the jurists, and it is valid for a public entity. If one bequeaths a third of their wealth or less for charitable works, their will is valid and must be executed. Sheikh al-Islam Ibn Taymiyyah holds the view that it is permissible to change the condition of the waqf (endowment) to what is more beneficial, even if this varies with the changing times. As for Sheikh Ibn Uthaymeen, he believes that it is permissible to change the will to what is better if it is for an unspecified entity, such as mosques and the poor, but not if it is for a specified entity. Sheikh Ibn Jibreen was asked about the ruling on demolishing a mortuary built as a waqf (endowment) to utilize its space for expanding a mosque. He replied that it is not permissible to demolish it except with the permission of those who built it and convincing them to do so, while transferring the waqf to another mosque. If the executors of the will see that it is in the interest to convert the remaining funds of the will into an endowment, they are permitted to do so, and they would be the supervisors of the endowment, because they have the right to the will, so they are the rightful owners of the endowment, and they must follow the condition of the endower in overseeing the endowment. If they wish to be relieved of their responsibility from the endowment, the recourse in that is the Sharia judge to appoint a supervisor.