That no man transgress (to mē huperbainein). Old verb to go beyond. Final use of to (accusative of general reference) and the infinitive (negative mē), parallel to apechesthai and eidenai ktasthai above.
And wrong his brother (kai pleonektein ton adelphon autou). To take more, to overreach, to take advantage of, to defraud.
In the matter (en tōi pragmati). The delicacy of Paul makes him refrain from plainer terms and the context makes it clear enough as in 2Co 7:11 (tōi pragmati).
An avenger (ekdikos). Regular term in the papyri for legal avenger. Modern men and women need to remember that God is the avenger for sexual wrongs both in this life and the next.