What is the ruling on someone who swore by Allah not to buy his sister a gas cooker for her marriage, then obeyed his sick father and bought it for her?

You have done well in obeying your father, and you owe an expiation for breaking an oath, according to the Prophet, peace and blessings be upon him, who said: "Whoever takes an oath and then sees something better than it, let him do what is better and abandon his oath." And in another narration: "and expiate for his oath." The expiation is feeding ten poor persons, or clothing them, or emancipating a believing slave. If you are unable to do that, then fast three days.