Is it permissible for the heirs to purchase the portion of their father’s land that their mother endowed, and to deposit its value into building a mosque, knowing that selling it to non-heirs would harm them because it would divide the land?

The fundamental principle regarding a waqf (endowment) is adherence to the donor's conditions. However, it is permissible to alter these conditions for a predominant benefit if it becomes impossible to utilize the waqf as originally intended. This is permissible provided that the waqf was not designated for specific individuals, and that the alteration is carried out through a Shar’i (Islamic law) judge. The judge has the authority to order the sale of the waqf and its replacement with something more beneficial, such as selling land to heirs at market price to prevent harm to them.