What is the ruling on holding a book fair in the basement of a mosque that was formerly a mosque and was relinquished by the mosque committee to other associations, knowing that the proceeds of the fair will go to the library affiliated with the mosque and to the mosque committee?

If the mosque committee overseeing the endowment deems that relinquishing the lower floor for the mosque to move to a more spacious floor is in its best interest, then its relinquishment of it is permissible. Therefore, there is no objection to holding an exhibition on the aforementioned floor, because it is no longer a mosque, and thus buying and selling within it is not prohibited.