that no one should take advantage of and wrong a brother or sister in this matter; because the Lord is an avenger in all these things, as also we forewarned you and testified.
Matter --  Lenski says this is a business term, and Paul dealing with commercial matters; saying Christians must live pure moral live, sexually and ethically. The word is well translated “matter” and the context is to indicate what kind of “matter, dealing, business” is under consideration.