“The expiation for a vow is the same as the expiation for an oath.”
It is also authentically reported that he said:
“Whoever vows to obey Allah must obey Him, and whoever vows to disobey Allah must not disobey Him.”
Thus, if a person intends to vow an act of obedience to Allah, he must fulfil it. If he vows something that is not an act of obedience, he is not obliged to fulfil it. If it is forbidden, he may not fulfil it. However, if he does not fulfil a vow made to Allah, he must offer the expiation for an oath according to most of the early scholars. This is also the view of Ahmad and Abū Hanīfah. According to one view, this applies without qualification; according to another, it applies only when the vow serves as an oath.
The second form is when his intention is to urge or deter, or to affirm or deny something. This constitutes swearing an oath involving a vow, divorce, manumission, zihār (declaring one’s wife as forbidden to oneself as one’s mother), or a declaration of prohibition. Examples include saying: “If I do such-and-such, I am bound to perform Hajj and fast for a year; my property is to be given in charity, my slaves are free, and my wives are divorced.” This category falls under the rulings on oaths, as well as those on divorce, manumission, vows, and zihār. Scholars hold three views concerning it.
The first is that, if he breaks his oath, he is bound by whatever he stipulated, because he committed himself to the consequence upon fulfilment of the condition. Since the condition has been fulfilled, that consequence becomes binding, just as with a conditional vow intended as an act of devotion.
The second view is that this is not a legally binding oath, so breaking it entails nothing: neither expiation nor the stipulated consequence taking effect. This is because it constitutes swearing by other than Allah, and the Prophet, may Allah bless him and grant him peace, said:
“Whoever swears an oath must swear by Allah or remain silent.”
In another authentic narration:
“Do not swear except by Allah.”