What is the ruling on a man's broken promise regarding marriage, knowing that a tahlil marriage is forbidden, but this marriage is valid?

The agreement between a man and the first husband to perform a tahlil marriage is impermissible, and it is forbidden for him to fulfill his promise, as it constitutes assisting in a sin. If the intent behind the scenario mentioned in the question is that he agreed with him on the tahlil, but intended a true desire-based marriage at the time of the contract, then the contract is valid. However, if he agreed with him on the tahlil and did not intend a desire-based marriage, and did not state at the time of the contract that it was a tahlil marriage, then the marriage is void. A tahlil marriage is forbidden and void, whether he intended to keep her or separate from her, and whether this condition was stipulated in the contract or before it, or if there was something indicating it without explicit wording, or if he intended tahlil without the knowledge of the woman, her guardian, and the divorced husband. In matters of dispute, it is best to refer to the Sharia court.