Fulfilling an Oath in Accordance With the Intention

Question :

I took an oath that if Allah gave me a sum of money from His Bounty due to my hard work and striving, then I would set aside a sum from what He had given me to build a congregational mosque. When I made the vow I assigned a sum in my mind which I thought was sufficient for building the mosque. The years and days passed and Allah made my ambition a reality. I wanted to fulfill my vow. But the sum which I previously expected would be enough for the purpose of building the mosque in the currency of my country had become devalued and was no longer sufficient to build a mosque. The sum I assigned would surely not be enough to furnish a congregational mosque nor to build one, due to the devaluation of the currency. I am thinking that I should give this sum of money in charity to the needy, the poor and the indigent among my relatives or others, or to give it to a charitable organization for the building of a mosque which has not yet been completed. Is this permissible? Benefit me (with advice) and may Allah benefit you (with a reward).

Answer:

It is an obligation upon you to fulfill the oath by building a mosque, according to your ability. If you wished to build a congregational mosque in which the Friday prayer is held, you must do that, according to the words of the Prophet ﷺ :
"Whoever vowed to obey Allah, he should obey Him and whoever vowed to disobey Allah, he should not disobey Him.

You must strive hard until you are able to fulfill your vow in full. But if you intended a specific sum of money by your vow, then you are not obliged to do anything except this, according to the words of the Prophet ﷺ :
"Deeds are according to intentions and every person shall have what he intended. "

If you could not build a mosque with it, then share with others in building a mosque, in accordance with the Words of Allah, Most High:
"So keep your duty to Allah and fear Him as much as you can."

May Allah make matters easy for you and allow you to meet your obligations.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 418-419