What is the expiation for having intercourse with one's wife during her menstrual period, knowing that the husband is remorseful and repentant?

Sexual intercourse with a menstruating wife is forbidden. Whoever commits this act must repent by feeling remorse and resolving not to repeat it. Scholars differ regarding the obligation of expiation (kaffarah). The majority do not consider it obligatory, while the Hanbalis deem it obligatory, and it is a dinar or half a dinar, with the choice left to the individual, which is the preferred view according to us. The expiation is estimated at 4.25 grams of pure gold. Paying the expiation is not a condition for the validity of repentance; rather, it is a separate obligation.