Is it permissible for anyone to take books out of the mosque, and does a student of Islamic knowledge have an excuse for doing so if he cannot afford to buy them, and what is the ruling on someone who insists on taking them out?

The handling of books endowed to a mosque is governed by the conditions set by the endower. If the endower stipulated that they should not be taken out, then this condition must be adhered to. Anyone who violates it is sinful and must repent. If the endower permitted their loaning, then there is no harm in it. There is no difference in this regard between a student of knowledge and others, or between the poor and those able to purchase them. However, if the endower's condition leads to the obstruction of benefiting from the endowment, some scholars have held that it is permissible to contravene the endower's condition if there is an overriding benefit in doing so.