Is the husband's unintentional penetration of his wife during menstruation considered a sin, and what is its expiation if two dinars are paid as expiation for it?

There is no sin upon the spouses if menstruation began during intercourse and they did not realize it until after finishing, for error is overlooked; due to the Prophet's (peace be upon him) saying: "Indeed, Allah has overlooked for my Ummah error, forgetfulness, and what they are forced to do." There is no expiation required of them because among the conditions for the obligation of expiation upon one who has intercourse with his wife during menstruation are knowledge, remembrance, and choice. And whoever has intercourse with his wife without knowing she is menstruating owes no expiation.