The Ruling on Giving Charity Equal to the Value of the Vow

Question :

I vowed to dig a well in Allah's Cause, but I was not successful. Now I wish to change my intention to give in charity what I vowed to give in Allah's Cause. Would I (in that case) have fulfilled my vow or not?

Answer:

A vow must be fulfilled if it is an act of obedience to Allah, and if the one who makes the vow has the ability to fulfill his vow. Allah, Most High commanded the fulfillment of vows, and He will grant rewards for that. He Most High says:
"Then let them complete their prescribed duties (Manasik of Hajj) and perform their vows, and circumambulate the Ancient House (the Ka'bah at Makkah)

He also says:
"They (are those who) fulfil (their) vows."

And, the Prophet ﷺ said:
"Whoever vowed to obey Allah, he should obey Him and whoever vowed to disobey Allah, he should not disobey Him."

So this man is obliged to fulfill his vow by digging the well as he vowed, and it must be dedicated for the Muslims' use. If he is unable to do so, or he does not find a suitable location, or, he is prevented by the government, the leaders or his relatives, it is permissible for him to renounce that and perform the nearest thing to it that he can. If he is unable to, he must give the value of the cost of digging the well and its accompanying expenses in charity. The charity should be for the Mujahidin, the indigent and those in need. And Allah knows better.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 6 Pages 424-425