What is the ruling on a wife stipulating a condition in the marriage contract that includes the husband's agreement to her future work, knowing that the husband does not desire her to work, and is it permissible for the husband to threaten his wife with taking another wife if she works, knowing that the spouses love each other?

If the wife stipulates a condition outside the marriage contract, such as a benefit for her, it does not affect the validity of the contract. However, scholars have differed regarding the obligation to fulfill it. The majority of scholars held that it is not obligatory, while others held that it is obligatory to fulfill it. This latter view is the most preponderant, based on the Almighty's saying: (O you who have believed, fulfill [all] contracts) and the Prophet's (peace be upon him) saying: "The most worthy of conditions to be fulfilled are those by which you make intimacy lawful."

Therefore, you must fulfill this condition for your wife unless she willingly waives it. You can try to persuade her to waive it.