Why is the expiation for a conditional divorce vow the expiation for an oath (kaffarat yamin)? And does the husband offer expiation for his oath if he regrets it after his wife has fulfilled his condition?

A husband's statement to his wife: "If you go to such-and-such a place, then you are divorced," is considered a conditional divorce, and it takes effect upon her going out, according to the majority of scholars. Sheikh Al-Islam Ibn Taymiyyah, however, views that if the intention behind it was an oath, then it does not take effect, and an expiation for breaking an oath becomes obligatory. The preponderant opinion is that of the majority. If the wife does not go out, the divorce does not take effect. If the husband permits her to go out, he is bound by the divorce according to the majority. If he intended an oath and followed Ibn Taymiyyah, and offered an expiation for an oath, then he may offer it after or before breaking the oath.