What is the ruling on a person who makes a vow and instantly cancels it because he knows that he will be unable to fulfill it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

When a person makes a vow to do an act that draws him closer to Allaah, the Exalted, he is obliged to fulfill his vow. There is no disagreement among the scholars in this regard. Allaah, the Exalted, praised His slaves who fulfill their vows. He says (what means): {They [are those who] fulfill [their] vows and fear a Day whose evil will be widespread.} [Quran 76:7] Allaah, the Exalted, also says (what means): {… and fulfill their vows…} [Quran 22:29] Moreover, ‘Aa’ishah reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to do an act of obedience to Allaah, the Exalted, must do so, and whoever vowed to disobey Allaah, the Exalted, should not disobey Him." [Al-Bukhari, Ahmad, Abu Daawood, An-Nasaa’i, At-Tirmithi and Ibn Maajah]

Therefore, it is impermissible for you to cancel your vow after it becomes binding. However, if you are, really and honestly, unable to fulfill your vow, in a way that is not expected to end, then you should offer expiation like that of breaking an oath.

Allaah Knows best.