What is the ruling on a man saying to his wife: "By divorce, I will marry over you"?

This is an oath by divorce, and it is a conditional divorce according to the majority of scholars. If you do not do what you swore to do, the divorce occurs. This is contrary to Ibn Taymiyyah, who holds that an expiation for an oath is due.

If you did not specify a particular time for the marriage, then the divorce does not occur unless its realization becomes impossible, such as by death, for the conditional particle "إن" (in - if) does not necessitate a specific time, and thus the condition is fulfilled over time.

One must be wary of swearing by divorce because it is an innovation and among the oaths of the dissolute. It is not appropriate to use marriage to another woman as a means of threatening one's wife.