Is it a major sin to have intercourse with one’s wife during menses? Is there a severe warning concerning it?
If that ever happens by mistake, then, if one had intercourse in the begging of the menses, it is best to give one dinar in charity and if one had intercourse during the end of the menses, it is best to give half of a dinar in charity.
It is compulsory (wajib) to repent (tawbah) and seek forgiveness (istighfaar).
[And Allah Ta’ala knows best]
6th Jamadul Ulaa 1340 AH
(Hazrat Mufti Maulana) Zafar Ahmed Usmani (May Allah have mercy on him)
Imdadul Ahkam Vol.1 Pg.362
 Allah Ta’ala said, “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 Allah has ordained for you. Indeed, Allah loves those who are constantly repentant and loves those who purify themselves.’” (Surah Baqarah: 222)
 Abu Hurairah narrated that the Prophet ﷺ said, “Whoever engages in sexual intercourse with a menstruating woman, or a woman in her anus, consults a soothsayer, then he has disbelieved in what was revealed to Muhammad.” [Tirmidhi #135]
 Mustahab (desirable). This is the opinion of the Hanafiyah and Shafi’eyah. As for the Hanabilah, they say that it is wajib to pay half a dinar. The Malikiyahs say that there is no kaffarah (monetary recompense). [See Mawsoo’ah Fiqhiyyah Vol.18 Pg.324]
 One dinar is 4.374 grams of gold.
 Half of a dinar is 2.187 grams of gold.
 Ibn Abbas رضي الله عنهما narrates that the Prophet (صلى الله عليه وسلم) said about a man who has sexual intercourse with a menstruating woman, “He should give one or half dinar as sadaqah.” [Collected by Abu Dawud #2168] Ibn Abbas رضي الله عنهما narrated that the Prophet صلى الله عليه وسلم said, “When the blood is red then (give) a Dinar. And when the blood is yellow then half Dinar.” [Collected in Tirmidhi #137]