What is the expiation for someone who swore an oath of divorce upon his wife that she would not do something, but the oath was not actualized?

If the wife breaks an oath and does what the husband swore she should not do, divorce occurs, according to the majority of jurists, and this counts as one pronouncement of divorce. If it is not the third pronouncement, he may take her back before the 'iddah (waiting period) expires without a new contract, or with a new contract after it expires. It is advised to avoid uttering divorce in the future.