What is the expiation for having intercourse with one's wife while she has not yet purified herself from menstruation, and a little yellowness remains?

It is impermissible for a Muslim man to have intercourse with his wife before she has purified herself and performed ghusl (ritual bath) after menstruation. This is based on the Almighty's saying: "And they ask you about menstruation. Say, 'It is an impurity, so keep away from women during menstruation. And do not approach them until they are pure.' But when they have purified themselves, then come to them from where Allah has commanded you." (Quran 2:222) Whoever does this must seek Allah's forgiveness and repent. Some scholars have held the view that one must give in charity half a dinar. A dinar is equivalent to 4.25 grams of gold.