Is there expiation upon one who had intercourse with his wife during days when she suspected the onset of menstruation, and what is it?

Sexual intercourse during menstruation is a major sin that necessitates expiation. What the questioner saw has two scenarios: Either it is a discharge other than blood, in which case there is no expiation; or it is blood. If it is blood seen after fifteen days of purity, then it is menstruation; otherwise, it is not. However, if she saw it while she was menstruating, the default is that the menstruation continues until she sees purity, and the husband must repent and offer expiation. If it is merely an expectation of blood discharge, then she is pure, and her husband is permitted to have intercourse with her.