Is expiation obligatory for one who had intercourse with his wife while she was in a state of post-natal bleeding (nifas)—without ejaculation—due to ignorance of the ruling, and is it permissible to pay it in installments?

Sexual intercourse with a woman in postpartum bleeding (nifas) is forbidden because postpartum blood is considered menstrual blood, and we know of no disagreement on this matter. Expiation (kaffarah) is not obligatory according to the majority of scholars; what is obligatory is repentance. If you wish to be cautious by offering it, the expiation is one dinar or half a gold dinar (4.25 grams of gold). The prohibition of intercourse includes not ejaculating. If we were to say that expiation is obligatory, it would be immediately due because its cause is a sin. Your not knowing that expiation is obligatory is not an excuse as long as you knew of the prohibition.