What is the ruling of Islamic law regarding a vow not to approach zinā (fornication/adultery), and then failing to fulfill it, despite seeking forgiveness and repentance?

Adultery is one of the grave sins and immoral acts. For someone who vows not to commit adultery, the majority of scholars hold that their vow is not binding because adultery is forbidden by Islamic law. However, the Hanbalis maintain that the vow is binding and that expiation is obligatory for anyone who violates it. What is obligatory is sincere repentance, regret, seeking forgiveness, and offering expiation. One should not despair of Allah's mercy.