The Prophet ﷺ said about a man who has sexual intercourse with a menstruating woman: He should give one or half dinar as sadaqah. (Using translation from Abū Dāʾūd 2168)
عَنِ النَّبِيِّ ﷺ فِي الَّذِي يَأْتِي امْرَأَتَهُ وَهِيَ حَائِضٌ قَالَ «يَتَصَدَّقُ بِنِصْفِ دِينَارٍ»