What is the ruling on breaking an oath of divorce that I regret, and is expiation (kaffarah) obligatory for it? If so, what is its amount?

The majority of scholars hold that swearing an oath by divorce results in the divorce taking effect upon the fulfillment of the stipulated condition, even if the husband did not intend for it to occur. As for Shaykh al-Islam Ibn Taymiyyah, he holds that divorce does not occur in this case. However, the fatwa we issue leans towards the opinion of the majority. If, however, the husband intended to bring about the divorce upon the fulfillment of the condition, then it occurs by scholarly consensus.

One must be cautious not to be lenient in swearing by divorce. If there was a reason that prompted the oath and that reason is no longer present, then there is no harm in buying the dress, and nothing will result from the oath in that case.