Is a wife considered sinful if she invites her husband to intercourse, thinking that her menstruation has stopped, but then blood appears when her husband touches her?

If your menstrual bleeding stops and you observe signs of purity, then there is no harm for your husband to have intercourse with you, nor for you to invite him to it, even if bleeding occurs again afterwards without you noticing it. However, if you rushed to confirm purity, then you must repent and seek forgiveness. Purity is confirmed by seeing the white discharge (al-qassah al-bayda') or complete dryness.