Is it permissible for a woman to ask her fiancé, who is married and whose previous wife returned to him only for the sake of the children, for an assurance or a promise that he will not go back to his first wife in the future, knowing that this was previously agreed upon?

If the wife stipulated in the marriage contract that he should not marry another, and he then does marry, she has the right to annul the marriage. However, if there was no such stipulation in the contract, but merely a promise before the contract, she does not have the right to annul it. The wife does not have the right to stipulate to her husband that he should not be just to his other wife, or that he should be less just to her, because such conditions are invalid and contradict the essence of the contract.