What should I do about the book "Al-Tafsir Al-Muyassar" (The Simplified Interpretation) that I bought, which bears the stamp of a waqf (endowment) of the Prophet's Mosque, given that the bookstore refuses to take it back and claims to have purchased it from another mosque? Is it permissible for me to keep it and read from it?

If the mosque administrators sold the books for the benefit of the mosque based on a fatwa from some scholars, then there is no harm for you in keeping them, reading them, or lending them. A group of scholars, including Sheikh al-Islam Ibn Taymiyyah, permitted the sale of waqf (endowment) for the sake of benefit, and he favored this view. He supported it by citing the action of Umar ibn al-Khattab, may Allah be pleased with him, when he ordered the relocation of the Kufa mosque and placed the treasury in its qibla, and this was in the presence of the Companions, and they did not object to him.

Al-Ruhaibani mentioned that Sheikh Taqiyuddin stated that if it is permissible to replace an endowed mosque with another more beneficial and suitable one, even if its benefit has not completely ceased, then it is even more appropriate to permit the replacement of a waqf designated for exploitation, especially since its sale for necessity is permissible according to Ahmad. Similarly, the sale of waqf for necessity is permissible for an equivalent item, and without necessity, it is permissible for something better for the sake of benefit. The relocation of the Kufa mosque indicates the permissibility of replacement when the exchange is more advantageous, even if the waqf is not rendered useless, as long as there is a clear benefit.