What should a person do regarding land upon which he vowed to build a mosque, and agricultural land revenues designated for the same purpose, after his neighbor built a mosque next to the location he specified, and the latter refused to help him with the construction?

The one who made a vow must sell the land that he vowed for building a mosque in a place where another mosque already exists, and buy land with its price in another place that needs a mosque, and build it there, in fulfillment of the vow and for the benefit of Muslims. Ahmad ibn Hanbal permitted replacing one mosque with another for the sake of benefit, citing the action of Umar ibn al-Khattab, may Allah be pleased with him, when he replaced the old Kufa mosque with another, and also when he allowed moving a mosque to another village if the place became desolate. Moreover, it is permissible to sell an old mosque and build another mosque with its price in another village if there is no longer a need for it in the first village, considering the general welfare of Muslims.