Is a wife forbidden to a man who has had anal intercourse with her, and what is the expiation for this act, especially if he has repented and regrets it, but still sees it in his dreams?

Performing intercourse with one's wife in her anus is one of the gravest sins. However, it does not necessitate حرمة (prohibition) between the spouses. The one who commits it must repent. There is no expiation for it other than repentance and an abundance of good deeds. As for the dream, it indicates genuine remorse if the matter has consumed the thoughts of the dreamer.