What is the ruling concerning the plot of land adjacent to the mosque that previously belonged to the church, then became an endowment for the mosque, and a person seized it and registered it in his name, and now wants to sell it to the mosque committee at a high price?

This issue is decided by the Sharia judiciary, and fatwa has no دخل (entry) into it except to clarify the Islamic ruling. The ruling is that if it is proven that this land is a waqf (endowment), it is not permissible for anyone to transgress against it, seize it, or usurp it, based on the saying of the Prophet, peace be upon him: "Whoever usurps a handspan of land will have it encircled around his neck from seven earths on the Day of Resurrection." And whoever has usurped something must return it, based on the saying of the Prophet, peace be upon him: "The hand is responsible for what it takes until it returns it." He also owes the rent for this house or land from the time he usurped it until the time he surrenders it.