Is having intercourse with one's wife after her menstruation has ended, only to discover brown discharge during intercourse, considered a sin that necessitates expiation or obliges the wife to do anything?

The brownish discharge seen by the wife after the end of her menstrual period, which is not connected to blood, is not considered menstruation. Therefore, the husband was not obliged to refrain from intercourse with her. Consequently, there is no sin upon the spouses nor any expiation due from them, even if it later becomes apparent that she was menstruating and they were unaware of it, based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended."