Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is acceptable for a woman to ask for a term added to a marriage contract that her husband cannot take a second wife. Once the husband has signed to it, he is bound to comply with it.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Inserting conditions in marriage contract
It is acceptable for spouses to stipulate conditions in marriage contracts as long they are not contrary to the teachings of Islam and opposed to the main objective of marriage.
Preventing husband from taking second wife in marriage contract: Valid?
Permission to marry a second wife is only an exception; in other words, it is an allowance given because of some exceptional conditions. Therefore, no one would say it is prescribed or open to everyone regardless of circumstances.
Therefore, it is acceptable for a woman to ask for a term added to a marriage contract. Once he has signed to it, he is bound to comply with it. Should he break the pledge has signed, she would have the freedom to be released from the marriage contract.
That is the view of eminent scholars of the past and present, including Imam Ahmad ibn Hanbal.
The Prophet (peace be upon him) said, “Muslims ought to fulfill the terms of their contracts.” (Ad-Daraqutni)
And he further said, “There is no contract more worthy of compliance like the terms of a marriage contract.” (Al-Bukhari)
Almighty Allah knows best.