What is the ruling of Islamic law on fulfilling a vow to build a small mosque on a piece of land, and then it becomes apparent that there is no need for it due to the abundance of surrounding mosques; and is it permissible to fulfill the vow by donating to build another mosque?

A vow to build a mosque must be fulfilled if the location is in need of one, because building mosques is among the acts of obedience for which a Muslim is rewarded after death, as the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him," and he also said: "Indeed, among the good deeds and merits that follow a believer after his death... or a mosque he built."

However, if the place designated for building the mosque does not need another mosque, such that its construction would lead to its abandonment or the abandonment of existing mosques, then a specialized real estate authority should be consulted to assess the value of the place. Then, that value should be disbursed to purchase another place in a location that needs a mosque, and the mosque should be built there according to the vowed specification.