What is the ruling on breaking a covenant with Allah upon discovering that what was covenanted upon is permissible?

If a Muslim makes a covenant with Allah to abandon something he believed to be forbidden, but then it becomes clear to him that it is permissible, then that is an oath. He is permitted to break it and offer expiation for his oath. In fact, it is better for him to go back on it if doing so is better, for the Prophet, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do what is better and offer expiation for his oath."