What is the expiation for financial dealings with a friend after having sworn not to deal with him, saying: "My wife be unlawful to me, like the back of my mother"?

You must repent from ẓihār because it is forbidden, reprehensible, and a falsehood, and some jurists have considered it a major sin. If you dealt with the man, then you owe two expiations: an expiation for breaking an oath, and an expiation for ẓihār, because your statement: "My wife shall be unlawful to me as my mother's back if I deal with him" is considered a conditional ẓihār. You must perform the expiation for ẓihār before touching your wife, which is: freeing a slave, and if you cannot find one, then fasting for two consecutive months, and if you are unable to do that, then feeding sixty poor people.