She vowed to fast for a month if Allaah caused her husband to mend his ways

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Check below answers in case you are looking for other related questions:
A vow to do something that Allah has made obligatory and enjoined upon people is not valid
He pledged to give some money in charity and he does not have that money now

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Related Answers:
He swore an oath cursing himself if he smoked again
Can he pay money for kafaarat al-yameen because nowadays it is more beneficial than food?
He said to his wife: “You are to me like my mother”
A vow to do something that Allah has made obligatory and enjoined upon people is not valid
He pledged to give some money in charity and he does not have that money now

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