Charles Ellicott Commentary Colossians 3:22-25

"Servants, obey in all things them that are your masters according to the flesh; not with eye-service, as men-pleasers, but in singleness of heart, fearing the Lord: whatsoever ye do, work heartily, as unto the Lord, and not unto men; knowing that from the Lord ye shall receive the recompense of the inheritance: ye serve the Lord Christ. For he that doeth wrong shall receive again for the wrong that he hath done: and there is no respect of persons." — Colossians 3:22-25 (ASV)

Compare throughout Ephesians 6:5-9. The only peculiarity of this passage is the strong emphasis laid on “the reward of the inheritance.” “The reward” is in the original, a perfect recompense or requital. The “inheritance” is exactly that which no slave could receive; only a son could be an heir (Galatians 4:7). Therefore, the slave on earth is recognised as a son in heaven. He “serves the Lord,” but his service is the perfect freedom of sonship.

He who does wrong is clearly here the master , though, of course, the phrase cannot be limited to him.