Is it permissible to tie up the principal of houses and make their usufruct available to the heirs according to the share of each heir, and then to their male descendants and to whoever needs housing (as a charitable endowment), in order to prevent their sale and secure housing for the heirs after the death of the inheritor?

There is no harm for the questioner in endowing his houses to his heirs if the endowment is immediate, meaning that they are given full possession of them, and he does not make it contingent upon his death. However, if he makes it contingent upon his death, it then falls under the ruling of a will, and it is not permissible to endow it upon an heir. If he endows it to those who are not his heirs, the endowment must be within the limit of one-third, and anything exceeding that is not enforceable unless the heirs permit it. It is recommended to divide the endowment among the children according to the division of inheritance, with the male receiving the share of two females.