If a Muslim breaks his vow or oath he has to offer expiation. In case one has broken their oath, then they should feed ten needy persons. If it is not possible, then one may fast three days either successively or otherwise.
The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “If someone makes nadhr (a vow) to do an act of obedience to Allah, he or she must fulfil it. However, if a Muslim makes a nadhr to do anything unlawful such as committing a sin or cutting relation with one’s kinship, then he has to break his nadhr and offer expiation for it, the same way he does for breaking an oath. This is because making nadhr to do what is mubah (permissible) is subject to the same juristic ruling that applies to taking an oath.
This is based on the hadtih of the Prophet (peace and blessings be upon him) that reads: “The expiation of (breaking) a vow is similar to the expiation of (breaking) an oath.”
the aforementioned hadith is reported by Muslim and is only confined to making a vow to do the mubaah. The Prophet (peace and blessings be upon him) is also reported to have said: “Whoever vows to do an act of obedience to Allah must fulfill it.”