What is the ruling on a person selling a plot of land belonging to a mosque for half its price, claiming that its proceeds will go back to the mosque, while the benefit to the mosque is not clear to everyone, and is it permissible to purchase this land?

The fundamental principle is that land belonging to a mosque is an endowment (waqf) and it is not permissible to sell it or dispose of it in a way that removes it from its endowed status. This is the view of Malik, Al-Shafi'i, and Ahmad in one of the two narrations, who cite as evidence the hadith of Umar, may Allah be pleased with him, regarding the land of Khaybar: "Its اصل (root/substance) is not to be sold, nor inherited, nor gifted."

Ahmad, in another narration, held the view that it is permissible to sell or exchange an endowment if its benefits are completely ceased or if a greater interest necessitates it. Their evidence for this is the action of Umar, may Allah be pleased with him, who moved the mosque of the Date-Sellers and established the treasury (Bayt al-Mal) in its place. This occurred in the presence of the Companions and was akin to a consensus. This opinion takes into consideration prevailing interests and is the view adopted by Shaykh al-Islam Ibn Taymiyyah and Ibn Qudamah.

The authority responsible for selling the endowment is the ruler, or the specific supervisor if one exists.