A person can choose to sacrifice one of their rights and this is not considered haram in Islam; it is a matter of preference. The prophet (SAW) chose not to eat the Dhabb but he did not make it haram upon himself. Similarly, in Islamic contracts both parties are allowed agreeing to any condition that they like as long as two conditions are met: 1- It is not a condition that is prohibited in the sharee’ah. 2- It does not go against the spirit of the current contract. This is the official opinion of the hanbali madhab, and was also the opinion of Ibn Taymiyyah, ibn Al-Qayyim, and others. This is based upon the fact that Ali (RA) chose not to marry another woman while he was married to Fatimah (RA), which he abided by.
Ibn Qudamah (RH) said:
“If he stipulates that he will not take her out of her house or her city, or that he will not travel with her or will not take another wife, then he is obliged to fulfil that, and if he does not do so, then she has the right to annul the marriage. This was narrated from ‘Umar, Sa’d ibn Abi Waqqaas and ‘Amr ibn al-‘Aas (may Allah be pleased with them).”
Shaykh Ibn ‘Uthaymin (RH) said:
“If she stipulates that he should not take another wife, this is permissible. Some of the scholars said that it is not permissible, because it is restricting the husband in something that Allah has permitted to him, and it is contrary to the Qur’aan in which it says (interpretation of the meaning): “then marry (other) women of your choice, two or three, or four” [al-Nisa’ 4:3]. It may be said in response to that that she has a reason to ask him not to marry another wife and she is not transgressing against anyone. The husband himself is the one who is giving up his right; if he has the right to marry more than one, he is giving it up. So what is to prevent this condition being valid?
Hence the correct view with regard to this matter is the view of Imam Ahmad (may Allah have mercy on him), which is that this condition is valid.”
Al-Sharh al-Mumti’, 5/243
Shaykh Salih al-Fawzan (HH) said:
“Among other conditions that are valid in marriage is if she stipulates that he should not take another wife. If he fulfils the condition (all well and good), otherwise she has the right to annul the marriage because of the hadeeth, ‘The conditions which most deserve to be fulfilled are the ones that make marital intimacy permissible for you’ ”
Al-Mullakhas al-Fiqhi (2/345, 346)
Allah knows best.
-Source Shaykh Navaid Aziz