What is the ruling on purchasing an endowed land that was sold due to financial hardship or by the heirs, and is the buyer sinful for using or selling it?

Summary of the Answer:

An Islamic endowment (waqf) cannot be sold, gifted, or inherited. Once something is endowed, it leaves the ownership of the endower, and he cannot retract it, even if he comes to need it. The purchase of an endowment is invalid by consensus. Anyone who seizes an endowment and sells it is an usurper, whether he was the original owner before the endowment or not, and he must return it or its equivalent. The ultimate responsibility for compensation falls on whoever intentionally committed the aggression. Consequently, your father's purchase of the endowment is invalid, and he must reclaim the price from the seller, and the endowment should be restored to its original state.