Will a person who made a covenant with Allah to fast a day, other than Monday or Thursday, if they commit a certain sin, be rewarded for it?

Compelling oneself to a specific act of obedience to prevent falling into sin is a known practice among the early Muslims (Salaf). Likewise, increasing good deeds after committing bad ones to erase the effects of sin is an established principle in Islamic law. Allah Almighty says: ﴿Indeed, good deeds do away with ill deeds﴾ (Quran 11:114), and the Prophet ﷺ encourages following a bad deed with a good deed to erase it.

Fulfilling a vow earns a Muslim reward and leads to Paradise, as stated in Allah's Almighty words: ﴿They fulfill [their] vows and fear a Day whose evil will be widespread.﴾ (Quran 76:7). Fasting to fulfill a vow is a good deed for which its doer is rewarded.

As for the reward of fasting a day "in the way of Allah" mentioned in the noble Hadith: "Whoever fasts a day in the way of Allah, Allah will distance his face from Hell by seventy autumns," it can bear two meanings: 1. Jihad: This is the most common meaning of the phrase "in the way of Allah." 2. Sincerity to Allah Almighty: Meaning that the fasting is purely for the sake of Allah, whether it is during jihad or not.

In conclusion, the virtue of fasting a day in the way of Allah certainly includes whoever fasts with sincere intention during their jihad, and the bounty of Allah Almighty is vast, encompassing all acts of obligatory fasting. A servant is rewarded for fulfilling a vow and for performing acts of obedience.