Regarding A Woman Stipulating That Her Husband Not Take Another Wife
Note: READ CAREFULLY, THOROUGHLY, MORE THAN ONCE AND PAY ATTENTION TO THE THIRD FOOTNOTE!!!
What a Woman may Stipulate in a Marriage Contract
Written By: Imaam Ibn Qudamah al-Maqdisi
Translated By: Yahya Adel Ibrahim From al-Mughni of Ibn Qudamah Vol. 9, Page 483: Issue #1141
He said: "If a man marries her and (accepted) her stipulations that he shall not remove her from her home or city (country), then her stipulation is to be honoured/ fulfilled (from that moment on) due to what has been reported from Rasulullaah (sallallahu 'alayhi wa sallam). He said: "The most deserving of conditions to be honoured/fulfilled are those which the genitalia are deemed Halal by them (Marriage is built upon their acceptance)."
As well if he marries her and (accepts) her stipulation that he will not marry another woman while with her then she is granted the power to abandon him if he marries another."
The summary of (this issue) is that stipulations relating to Nikaah (pre-nuptiual agreements) are of three main categories.
(Translators note: Only the first issue is translated):
First: (A condition) that which must be fulfilled. It is a condition wherein its benefit and worth are returned. Examples are stipulations wherein he (vows) not to remove her from her residence or land, or that he will not travel with her (to foreign lands), or that he will not take another wife while with her. All of these are conditions that he would be bound to fulfilling (if he agreed to them before cohabitation). If he does not honour the stipulations she is granted the ability (and right) to annul the marriage (at her will). This is reported as being the opinion of:
‘Umar bin al-Khattab, Sa‘d bin Abi Waqas, Mu‘awiyah, ‘Amr bin al- ‘Aas (radia Allahu ‘Anhum). It was also stated by:
Shurayh, ‘Umar bin ‘Abdul-Aziz, Jabir bin Zayd, Tawus, Al-Awza‘i and Ishaq."
Those who deemed these conditions unacceptable were:
az-Zuhari, Qatadah, Hisham bin ‘Urwa, Malik , al-Laith, ath-Thawri, ash-Shafi‘i (in part), ibn al-Mundhir, and the people of logical deduction and inference.
Abu Hanifah and ash-Shafi‘ee stated: "If he breaks one of the aforementioned conditions she is not entitled to leaving him. The original dowry (Mahr) is deemed invalid and she deserves another equal Mahr to what she has already been given (by her husband)."
They use as their proof the statement of Rasulullah (sallallahu 'alayhi wa sallam):
"Any condition, which is not in Allaah’s Book, is worthless (invalid). Even if there are a hundred conditions. Allaah’s decision is more valid and Allaah’s condition is more binding." (Agreed upon)
(They say) such a condition is (not sanctioned) in Allaah’s Book since the Law does not espouse it.
He (sallallahu 'alayhi wa sallam) also said, "Muslims are bound to their conditions/stipulations. (But) a condition that makes Halaal a Haraam or a Haraam Halaal is (not from binding conditions)." (Bukhari, Tirmidhi and others)
This (type of condition) is turning a Halaal into a Haraam. Taking more than wife and traveling (are Halaal). As well, these conditions do not benefit or improve the ‘Aqd (marital pre-nuptial contract) and are not integral to it. In fact it would be similar to stipulating that she is not to present her self (ever) to him (in copulation)."
End words of Shafi‘ee and Abu Hanifah
For us (our understanding) we have the following (evidence):
"The most deserving of conditions to be honoured/fulfilled are those which the genitalia are deemed Halaal by them (Marriage is built upon their acceptance)."
As well his statement (sallallahu 'alayhi wa sallam): "Muslims are bound to their conditions/stipulations." (Previously referenced)
As well we have named many Sahaba who validated these stipulations, and none from their generation/peers refuted them. Therefore we establish this as being Ijma‘ (consensual agreement by them all).
It is narrated by al-Athram with his chain of narration that a man married a woman and (agreed to the stipulation) that she may reside in her (own) home. Thereafter he sought to transfer her elsewhere. They turned to ‘Umar (Radee Allahu 'anhuu) in disputation.
He (Radee Allahu 'anhuu) said, "Her condition is to be fulfilled."
The man replied, "If that is the case then she is to grant the divorce (he means that she is to relinquish her condition or divorce him)."
‘Umar (radee Allahu 'anhuu) replied, "The absolute rights are (established with their) stipulations (His right is over-ruled by his agreement to her stipulation)." 
This (as well as the other mentioned stipulations are) valid because there is only benefit found in it (i.e. it does not call to deviance) and it is not intended to depreciate or tarnish what is established by Nikaah. Therefore fulfilling the condition is obligatory. (These types of stipulations) are the same as if she stipulated that he increase her Mahr (from his initial offer) or not take her abroad.
As for the Prophet's (sallallahu 'alayhi wa sallam) statement: "Any condition, which is not in Allaah’s Book, is worthless (invalid)."
This is regarding a condition that is not founded upon Allaah’s Hukm and Sharee'ah. These (conditions) are substantiated in Sharee'ah. We have shown precedence for it. Those who seek to oppose this (valid opinion) must bring forth (explicit) evidence that renders the aforementioned evidence nullified.
As for their (Shafi‘i and Abu Haneefah) statement: that these stipulations make a Halaal Haraam, then we say that they do not make the Halaal into Haraam.
It merely ensures the right of a woman to decide for herself if she wishes to remain in that situation or not considering that he has not fulfilled his pre-marital agreement.
As for their statement: That there is no benefit or improvement in these stipulations.
We say that we object to this understanding. These (stipulations) are very beneficial for the woman. The benefit of the stipulator is beneficial as well for the one being stipulated to (since the man knows what is acceptable and what is not).
Fasl (Side point):
If she was to stipulate that he was to divorce his other wife then that condition would be deemed invalid and unacceptable.
Abu Huraira (radee Allahu 'anhuu) reports: "An-Nabi (sallallahu 'alayhi wa sallam) forbade a woman from making it a condition for her husband to divorce her sister (his other wife)." (Reported by al-Bukhari)
Verification of Athar:
 The Madh’hab of Malik validates a stipulation upon a man in which it is stipulated that he is not to marry another women while in marriage with the first (wife) or else she will be given the order (power) in her hand (to annul the marriage). She would be given the power to separate from him at her choice. This position of Malik is similar in meaning to the position of al-Imaam Ahmed. (Fatawa of Ibn Taymiyyah 32nd Volume, Page 164-165)
 Reported by Ibn Abi Shaybah in his Musaanaf (Vol. 4 pages 199). As well it is found in the Sunnan of Sa‘eed bin Mansour (Vo1. 1, Pg. 185)
 It is important to note that the second wife is legal for the man to wed. The issue is whether the first wife wants to remain in that situation or not. Many people mistakenly think that this stipulation makes it illegal for the husband to take the second or third wife. That is not the case at all. It is merely to ensure that the first wife is able to leave the marital relationship if she cannot deal with the new situation.
Mizazeez Note: You know subhanAllah, this whole explanation is really actually in some ways a beautiful reconciliation between the two different opinions. for while it supports a woman being able to stipulate that her potential husband not take another wife while being married to her, it also makes clear that her stipulating this does not "prevent" him from practicing something Halal (polygyny), but rather it allows the woman to stay or leave if he chooses to do so. so basically, both the man and the woman's rights are being addressed and safeguarded. masha'Allah, i love the Salaf. they were so on it. may Allah reward them. Ameen.