The issue of whether stipulations can be inserted into a marriage contract by the parties is a topic that has been debated by classical Muslim jurists and modern scholars. Some argue that such stipulations are permissible, while others argue that they are not.
Those who argue that stipulations are permissible cite the hadith in which the Prophet Muhammad (peace be upon him) allowed a woman to stipulate that her husband could not take another wife during their marriage. They also point out that there are many instances in Islamic history where stipulations were included in marriage contracts.
On the other hand, those who argue that stipulations are not permissible cite the principle that marriage is a contract of mutual consent and agreement, and that stipulations can undermine the essential nature of that agreement. They argue that the parties should not be allowed to insert stipulations that go against the basic principles of Islamic marriage, such as the rights and obligations of the spouses.
Modern scholars have also weighed in on this issue. Some argue that stipulations are permissible as long as they do not violate any Islamic principles, such as prohibiting polygamy or setting conditions for divorce that are not recognized by Islamic law. Others argue that any stipulation that goes against the basic principles of Islamic marriage is not valid.
In my opinion, the admissibility of inserting stipulations in a marriage contract by the parties should be based on a case-by-case basis, and the validity of the stipulation should be determined by whether it goes against any Islamic principles. If a stipulation does not violate any Islamic principles and is agreed upon by both parties, then it should be permissible. However, if a stipulation goes against any Islamic principles, it should not be allowed.
Ultimately, the purpose of a marriage contract is to establish the rights and obligations of the spouses in accordance with Islamic principles. If a stipulation helps to achieve that goal and is not in conflict with Islamic law, then it should be allowed. However, if it undermines the essential nature of the marriage contract or violates Islamic principles, it should not be permissible.