Is expiation (kaffarah) obligatory for someone who vowed to break his phone if his friend, who won it in a bet, returned it to him, and then he sought God's forgiveness and retracted his vow?

A vow of wagering money is a vow of disobedience, due to the corruption and squandering of wealth that it entails, which is forbidden by Islamic law. Therefore, it is not permissible to fulfill it. Your obligation is an expiation for an oath, for the Prophet, peace and blessings be upon him, said: "There is no vow in an act of disobedience, and its expiation is the expiation of an oath." The expiation for an oath is feeding ten poor persons, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days.