It is Obligatory to Fulfill a Vow as it is

Question :

I vowed to Allah, Most High that if He cured my sick daughter, I would slaughter a sheep for Allah's sake. Now she has been cured, all praise and thanks be to Allah. Is it permissible for me to give charity for the value of the sheep or not, because the poor man prefers money to meat? Benefit me (with advice) and may Allah benefit you (with a reward).

Answer:

It is an obligation upon you to fulfill your vow. So you must slaughter the sheep as you vowed to, and give it as charity to the poor, in order to draw closer to Allah, Most High, and in obedience to Him to fulfill your vow. This is in accordance with the saying of the Prophet ﷺ:
"Whoever vowed to obey Allah, he should obey Him and whoever vowed to disobey Allah, he should not disobey Him."

Narrated by Imam Al-Bukhari in his Sahih, on the authority of the Mother of the Believers, 'A'ishah, may Allah be pleased with her.

It is not sufficient for you to give charity for the value of it; rather, you must slaughter a sheep as you vowed to do. If you intended for you and your family to eat from it, or to invite your neighbors and your relatives to eat it, you may do as you intended. It is not necessary to distribute it among the poor, according to the words of the Prophet ﷺ:
"Deeds are according to intentions and every person shall have what he intended. We advise you not to make a vow again, in accordance with the saying of the Prophet ﷺ :
"Do not make vows, because a vow does not prevent anything of Allah's Predestination being implemented, it is only a means of extracting something from the miserly.

(Agreed upon, on the authority of 'Abdullah bin 'Umar bin Al- Khattab, may Allah be pleased with them)

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 420-421-422