What is the ruling of Islamic law on marrying another man after filing a Khul' case but before the final verdict is issued?

What the young man did by proposing to you before your irreversible divorce from your husband is an abomination. It is not permissible to hint at proposing to a woman observing her revocable divorce, so how much more so to explicitly propose to a married woman? It is not permissible to go along with him or agree with him to marry, because you are still under the marital bond of your husband. So, be patient until your waiting period (iddah) from him ends, and after that, it is permissible for him or someone else to propose to you.