I had sexual intercourse with my wife while her menses was not completely over; she still had a few yellowish excretions. What is the due expiation for such a sin, or is it not regarded a sin?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam,  is His slave and Messenger.

It is impermissible for a Muslim to have sexual intercourse with his wife before her menstrual period is over and she has performed her due Ghusl after the end of her period; Allaah The Exalted Says (what means): {And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allaah has ordained for you.} [Quran 2:222] In the verse, {until they are pure} means the cessation of menstrual blood; {when they have purified themselves} means purified with water i.e. Ghusl.

If a husband had sexual intercourse with his menstruating wife, he should repent to Allaah The Exalted, seek His forgiveness, and resolve not to commit this sin again. There is no expiation for this sin according to the majority of Muslim scholars. The only action required is that the husband repents and asks Allaah The Almighty for forgiveness as mentioned.

It was reported on the authority of Ibn ‘Abbaas that he said, “If a husband had intercourse with his wife while she is menstruating, he should give out half a Dinar in charity." [Abu Daawood as Marfoo‘]

The Hanbali scholars ruled according to this narration and obliged a husband who had intercourse with his wife during her menstrual bleeding to pay expiation.

In summary, it is forbidden for a Muslim man to have sexual intercourse with his menstruating wife before her due Ghusl from menses, and if he did, then he has to repent and seek forgiveness. If the husband offered a Dinar or half a Dinar for charity, it would be fine.

Note: A Dinar is equivalent to: 4¼ (four and a quarter) grams of gold.

Allaah Knows best.