The Shaykh—may Allah have mercy on him—was asked:

A man swore an oath invoking divorce that he would not marry a certain woman, but then decided to marry her. May he do so? Another man married a woman and stipulated in the marriage contract that he would not marry another woman alongside her, but then did so. Does she have the right to choose whether to remain with him?

He replied:

All praise is due to Allah, Lord of the worlds. He may marry her, and no divorce takes effect if he does so, according to the majority of the early scholars. This is the position of al-Shāfiʿī, Ahmad and others.

If he stipulated in the marriage contract that he would not marry another woman alongside her, and that if he did, the decision would rest with her, then this condition is valid and binding according to the schools of Mālik, Ahmad and others. Whenever he marries another woman alongside her, the decision rests with her: if she wishes, she may remain with him, and if she wishes, she may leave him. And Allah knows best.