Paul to Philemon: I thank God that you have refreshed the saints.
I am sending your slave Onesimus back to you. I became his spiritual father and wanted him to stay with me. But I wanted that to be with your consent. If he wronged you, forgive him. If he owes you anything, I'll pay it. Receive him as you would me, not as a slave to be punished but as a brother in Christ. Return him to me if possible ("benefit/joy" in vs 20 is closely related to the name "Onesimus"). I hope to visit you soon. The grace of Jesus be with you.
How this is about Jesus
Paul is being Christ-like, here, interceding for His people, taking any punishment they deserve on himself (vs 18-19).
When we have sinned, as Onesimus appears to have (vs 18), the solution is a direct appeal for forgiveness and reconciliation.
Paul expects slavery to fade away over time, where the Gospel is obeyed. Christian love should change the relationship between master and slave. Because of the large financial investment, this may take time to disentangle, and so Paul deals carefully with Philemon about it.