Does the husband incur the expiation of ẓihār if he says to himself, "If I approach my wife during her menstruation, she is as forbidden to me as my mother's back," and then does so without his wife's knowledge? What is the current amount of the expiation? And is it permissible to have intercourse with the wife after her purification but before performing the expiation?

If the husband did not utter the declaration of ẓihār but merely intended it, then nothing is incumbent upon him. However, if he did utter it, such as saying, "You are to me as the back of my mother (i.e., unlawful)," then this is an explicit ẓihār for which expiation is obligatory, regardless of whether he intended divorce or not. The expiation for ẓihār is to free a slave before intimacy. If he cannot find one, then he must fast for two consecutive months before intimacy. If he is unable to do that, then he must feed sixty poor people. It is not permissible for the husband to have intercourse with his wife, concerning whom he made ẓihār, before he expiates for it.