What is the ruling on breaking an oath made by divorce, and is expiation obligatory for one who swore by divorcing his wife that she would not go to work, knowing that she was menstruating at the time of the oath?

A piece of advice to refrain from swearing by other than Allah, as it carries a severe warning. Swearing by divorce or emancipation is characteristic of the oaths of transgressors. What occurred from you is a suspension of your wife's divorce upon her going to work, so the divorce takes effect upon her going, and this is the view of the majority. There is an opinion that differentiates between the intention of divorce and the intention of prohibition. If you intended only prohibition and did not intend divorce, then divorce does not occur; rather, you are bound by an expiation for an oath. The wife's menstruation does not prevent the occurrence of divorce according to the most authoritative opinion. It is advised to refer to the Sharia court to rule on these matters.