Is it permissible to change a vow to purchase a Qur’an for the mosque into paying a sum of money to the mosque instead of the Qur’an, especially since the mosque is already equipped with Qur’ans and is in need of money?

The default is the obligation to fulfill a vow in the manner specified by the one who made the vow, as long as he is able to do so. This is the view of the majority of jurists. Therefore, the questioner must fulfill his vow by placing copies of the Qur'an in the mosque, even if other copies already exist there. Some scholars have permitted giving the value of the vow, so it is permissible to give its value in cash, especially if the mosque is in need of money for its renovation, lighting, or similar purposes, and it has a sufficient number of Qur'an copies.