What is the ruling of Islamic law regarding the disposal of the proceeds from the sale of a house built on land that was endowed to my grandfather and his descendants after him, given that the house has collapsed, and knowing that the endower's conditions stipulate that the house be renovated using a quarter of the rent, after which the rent shall be an endowment for my grandfather and his descendants, and in the event of their extinction, the rent proceeds shall go to the mosque of our master Al-Hussein?

The endowment (waqf) must remain in existence, and its terms, as stipulated by the endower, must be followed. It is not permissible to sell it or dispose of it in a way that removes it from its endowment status, unless it has fallen into ruin and its benefits have become entirely defunct, and its restoration and repair are impossible. In such a case, it is permissible to replace it with another. Sheikh Al-Islam Ibn Taymiyyah held the view that if there is a need, the endowment must be replaced with a similar one, and if there is no need, it is permissible to replace it with something better, as this demonstrates a clear benefit. If it becomes impossible to benefit from any part of it, the entire endowment may be sold.

If the endowed house's land was sold and a residential building was constructed on it, another property can be sought to be endowed with the price of the endowed house, and the new endowment should adhere to the condition mentioned in the question.

However, if the endowment is contingent upon death, a person is not allowed to endow more than one-third of their property, because it is considered a bequest. In this case, if the endower has heirs other than his wife, the bequest is not enforceable except within the limit of one-third, in addition to the wife's share. The remaining part of the inheritance is the right of the other heirs. The questioner should seek them out and deliver this right to them. If he despairs of finding them, he should spend their share on public interests, including the aforementioned endowment.