A man said to a woman, "your husband's daughter is the daughter of my brother." How is the man related to the woman daughter.

Father's brother

The man says, "Your husband's daughter is the daughter of my brother." This means that the woman's husband's daughter (i.e., the woman's stepdaughter) is the niece of the man. Since the woman's husband's daughter is the niece of the man, the man is the brother of the woman's husband, making him the woman's brother-in-law. But the question asks about the man's relationship to the woman's daughter, not the woman herself. Since the man is the woman's brother-in-law, he would be the uncle of the woman's daughter.

So, the man is  Father's brother of woman's daughter.