“I beseech thee for my son Onesimus, whom I have begotten in my bonds: Which in time past was to thee unprofitable, but now profitable to thee and to me: Whom I have sent again: thou therefore receive him, that is, mine own bowels:…. For perhaps he therefore departed for a season, that thou shouldest receive him for ever; Not now as a servant, but above a servant, a brother beloved, specially to me, but how much more unto thee, both in the flesh, and in the Lord?” (Philemon 10-12,15,16).
What did the Mosaic Law demand?
Moses declared to Israel in Deuteronomy chapter 23: “ Thou shalt not deliver unto his master the servant which is escaped from his master unto thee:  He shall dwell with thee, even among you, in that place which he shall choose in one of thy gates, where it liketh him best: thou shalt not oppress him.” The Mosaic Law could not be clearer: under no circumstances was the escaped servant to be returned to his master. Yet, as today’s Scripture bears out, the Apostle Paul sent runaway slave Onesimus back to owner Philemon. Why did Paul not follow Moses?
Due to an undisclosed wrongdoing, Onesimus ran away from Philemon’s house in Colosse. While Paul was imprisoned at Rome hundreds of miles away, Onesimus showed up there and Paul shared the Gospel of Grace with him. Onesimus trusted Jesus Christ as his personal Saviour and thus joined the family of God. Paul sent Onesimus back to Philemon, Onesimus carrying a special message—the Bible Book we now know as “Paul’s Epistle to Philemon.” In that little Book, the Apostle explained to Philemon the spiritual transformation of Onesimus.
As a former Pharisee and Jewish Law scholar (Acts 22:3; Philippians 3:5), Paul was very much aware of what Moses commanded. Still, the Law was not in operation. The Church the Body of Christ was under a whole new dispensation (set of divine rules). Grace would enable Philemon, no matter how badly he had been cheated, to accept Onesimus as an eternal “brother beloved” in Christ. By sending Onesimus back to Philemon, Paul afforded them both an opportunity to experience just how amazing (and unifying) God’s grace is! source