What is the ruling on a husband touching his wife's anus, perhaps with a superficial penetration unintentionally by both parties, and what is the expiation for that? Is direct contact with the anus from the outside permissible during menstruation and at other times?

It is forbidden to have intercourse with one's wife in her anus. However, pleasure between the buttocks without anal penetration is permissible, though it is disliked (makrooh) because it may lead to anal intercourse, and it must be avoided if one fears falling into the forbidden act. As for a man having foreplay with his wife in the anus during menstruation, under the condition mentioned, it is permissible based on the Prophet's (peace and blessings be upon him) saying: "Do everything except intercourse."