What is the ruling regarding someone who swore to his wife by triple divorce if his mother does not return to live with him, intending thereby not divorce, but rather to pressure his mother to change her mind?

The majority of scholars hold that a conditional divorce occurs if the condition is met, and this is the most preponderant opinion. If you conditioned your wife's triple divorce on your mother not returning to live with you, then either you specified a time for her return, such as a month or a year, in which case, if the time elapsed before her return, the oath is broken. Or, you did not specify a time, in which case the oath is not broken, and your wife remains under your marital bond until her return becomes impossible, such as if one of the parties dies. In the event the oath is broken, a triple divorce occurs according to the majority of scholars, and your wife becomes unlawful to you until she marries another man. However, according to the madhhab of Ibn Taymiyyah, the divorce does not occur; rather, an expiation for breaking an oath is due if you did not intend divorce.