Stipulating in the marriage contract that if the husband marries another woman the contract will be terminated
Only a husband can stipulate conditions whose fulfillment results in divorce. This is because it was reported by
`Amr ibn Shu`ayb from his father from his grandfather that the Prophet (peace be upon him) stated,
The son of Adam cannot take a vow (to give something) except in regard to something he already possesses; he cannot manumit (a bondsman) except those he owns, and he cannot divorce except (a woman) he already has (in marriage).
Abu Dawud, and
Al-Tirmidhy and ranked as a Hadith Hasan [a Hadith whose chain of narration contains a narrator with weak exactitude, but is free from eccentricity or blemish] by Al-Tirmidhi)
There are other narrations implying the same meaning. Thus, the marriage contract referred to is valid and the condition is invalid as it is a sort of divorcing a woman before concluding her marriage contract. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.