All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
A family Waqf is a binding contract whereby the endowed property is no more possessed by the person who had endowed it, and he can neither take it back nor change the purpose for which it was endowed, nor to whom it was endowed. The right to benefit from the endowed property is transferred to whom it was endowed for, and it is permissible for them to claim their legitimate right to benefit from it; because it is among the constant rights of the people. This is in addition to the fact that the one who had endowed it isn`t at liberty to dispose of it the way he likes; because the endowment got out of his possession.
" An endowment is a binding contract that can`t be taken back"{Al-Bahjah Al-Wardiah}. "Selling an endowment isn`t permissible."{E`anat Attalibeen}.
Moreover, Imam Ahmad reported in his Musnad the Hadith of Umar(May All be pleased with him): …….."If you wish you can keep it as an endowment to be used for charitable purposes." So, `Umar gave the land in charity (i.e. as an endowment on the condition that the land would neither be sold nor given as a present, nor bequeathed."
In conclusion, if there is legal and official proof that a property was donated as an endowment, then it gets out of the possession of the person who had donated it in the first place, so he isn`t permitted to take it back or dispose of it as his own; rather, its profits must serve the purpose for which it was endowed. And Allah knows best.