Is it permissible for a husband to threaten his wife with divorce during her menstrual period or during a period of purity in which he has had intercourse, believing that it does not take effect? And is it permissible for him to issue a fatwa to those who ask him that his divorce during those periods does not take effect, based on the opinion of some scholars, or must the questioner refer to scholars himself?

Know that the most preponderant of the sayings of the people of knowledge is that divorce occurs during menstruation or during a period of purity in which intercourse has taken place. The Sharia has provided means to discipline a wife in case of her defiance (nushuz). As for your informing about the fatwa of another person, there is no harm in that, by way of narrating it, if you are precise in what he said, because it is the نقل (transmission) of the Mufti's words for the questioner to act upon.