Is expiation obligatory for one who swears by saying: "You are forbidden to me until the Day of Judgment, this will not happen until such and such a time," and is the fiancée considered forbidden to him if the matter occurs?

If you uttered the phrase prohibiting your wife after the marriage contract, scholars differ regarding its ruling: some consider it zihar, some a divorce, and some an oath. If you intended divorce by it, then divorce occurs. If you intended zihar, you are liable for the zihar expiation upon returning [to your wife]. However, if you intended an oath or did not intend anything specific, then you must perform an expiation for an oath.

If you uttered the phrase before the marriage contract, then if you intended divorce or an oath by it, it is considered idle talk with no effect. If you intended zihar, the majority [of scholars] hold that it is idle talk, while some consider it zihar.