It was narrated from Miqsam, the freed slave of ‘Abdullah bin al-Harith, that Ibn ‘Abbas (رضي الله عنه) told him:

The Prophet (ﷺ) set (the expiation for) having intercourse with a menstruating woman at one dinar (to be given in charity. And if he has intercourse with her when the bleeding has stopped but she has not yet done ghusl, then (he should pay) half a dinar. All of that (was narrated) from the Prophet (ﷺ).