Sheikh Salman al-Oadah
Yes, it is permissible for the woman to set the condition, before the marriage is contracted, that her husband may not contract another marriage. This is a valid condition.
The basic ruling regarding contractual conditions is that they are permissible. A contractual condition only becomes invalid under two circumstances:
1. The condition violates a ruling of Islamic Law.
2. The condition contravenes the essential purpose or purposes of the contract.
If either of these two circumstances applies to a condition, then the contractual clause specifying the condition is an invalid clause.
An example of a clause that violates a ruling of Islamic Law would be a clause where the prospective wife stipulates the condition that her husband cannot attend congregational prayer. This clause is invalid since it violates an Islamic legal ruling enjoining upon men to offer their prayers in congregation.
Another example of a condition that violates a ruling of Islamic Law would be for the prospective wife to set a condition that her husband divorces his present wife. This condition is injurious to the present wife. It also goes against the ruling where the Prophet (peace be upon him) said: “A woman cannot ask for her sister to be divorced so she can have him to herself. She can have just what Allah has prescribed for her. ” [Sahîh al-Bukhârî (2723) and Sahîh Muslim (1408)]
An example of a clause that violates an essential purpose of the marriage contract would be a clause stipulating the condition that the man many never engage in sexual relations with his wife. Since conjugal relations are an essential aspect of a marriage, such a condition is invalid.
As for the condition that the husband refrain form contracting another marriage thereafter, this does not violate any Islamic legal ruling. Marriage to another wife is merely permissible in and of itself. The husband commits no sin by refraining from contracting a polygamous marriage. By stipulating this condition, the woman is not saying that polygamy is unlawful, but just that she wishes her husband to refrain from it.
And Allah knows best.
Source: Islam Today