If a husband made a verbal commitment before concluding the marriage contract not to marry another woman, is such a commitment binding?

As regards the issue posed,  the late prominent Muslim scholar and author of Fiqh As-Sunnah, Sheikh Sayyed Sabiq, He states the following:

“Some of the conditions that may be included in the marriage contract may be in favor of the wife, such as when the husband pledges not to drive the wife out of her house or town, nor to accompany her in any travel, nor to marry another beside her, etc.

Some scholars maintain that such a contract of marriage is valid, but the accompanying conditions are void, so the husband is not bound to fulfill these conditions. This is the view of Abu Hanifah, Ash-Shafi`i, and many other jurists. They quote the following proofs in support of their view:
– Allah’s Messenger (peace and blessings be upon him) is reported to have said: “Muslims are bound by their conditions, unless it be a condition which turns a halal (lawful act) into haram (unlawful act) or a haram into a halal.”

They maintain that the wife’s stipulating that her husband must not marry another beside her, or not to take her with him in his journeys, is null and void, because it makes a lawful thing, i.e. marriage or accompanying her in travels, unlawful.

– Allah’s Messenger (peace and blessings be upon him) is also quoted to have said: “Any condition which is not backed up by the Book of Allah (the Glorious Qur’an) is void, even there were a hundred conditions.” Scholars say, these conditions have no basis in the Book of Allah, so they should not be included in the marriage contract.

– They also say that these conditions are irrelevant as for concluding the marriage contract.

Another group of jurists are of the opinion that, if the wife has stipulated in the marriage contract that her husband should not marry another beside her, then if he does not fulfill that condition the contract will be null and void. This view is held by `Umar ibn Al-Khattab, Sa`d ibn Abi Waqqas, Mu`awiyah, `Amr ibn Al-`Aas, `Umar ibn `Abd Al-`Aziz, Jabir ibn Zayd, Tawus, Al-Awza`i, Is-haq, and the Hanbali jurists. They quote the following proofs in support of their opinion:

– Allah Almighty says, “O ye who believe! Fulfill your undertakings.” (Al-Ma’idah: 1)

– Al-Bukhari, Muslim and others reported on the authority of `Uqbah ibn `Amir that Allah’s Messenger (peace and blessings be upon him) is reported to have said, “The conditions you have the most duty to fulfill are those by which you have made marital relations lawful.” With his chain of narrators, Al-Athram reported that a man married a woman; it was stipulated in the marriage contract that he should provide her a certain house. Thereafter, the husband wanted her to dwell in another house, so the wife lodged a complaint to `Umar ibn Al-Khattab; `Umar said that the wife should be provided with the house stipulated in the marriage contract.

– They, moreover, maintain that the wife’s condition that her husband should not marry another beside her, is binding, for it contains a benefit for the wife and does not contradict the objectives of marriage; it is the same as the stipulation of increasing the dower. Ibn Qudamah gives preference to this view over the first; he says, “This was the opinion of the Companions, and we do not know any one who disagreed with them in their lifetime. In addition, the hadith that states: “Any condition which has no basis in the Book of Allah is void” means any condition which is inconsistent with the rules of Shari`ah, but such a condition is admissible, and we have mentioned the proofs that support its admission; therefore, he who doubts its admission should support his view with evidences.”

Based on the above fatwa, the wife has the right to stipulate in the marriage contract that her husband must not marry another beside her and it’ll be binding upon him. If the man marries another wife, then she is given the choice either to obtain divorce or to stay with him as a co-wife.