[Deu 23:15-16 NLT] (15) "If slaves should escape from their masters and take refuge with you, you must not hand them over to their masters. (16) Let them live among you in any town they choose, and do not oppress them.
[Phm 1:12 NLT] (12) I am sending him back to you, and with him comes my own heart.
Paul and Onesimus had a new relationship, by Onesimus' [eternal] rebirth. Philemon and Onesimus also were now brothers, so are the sending and keeping in that context? Onesimus evidently chose, whether returning or staying. Nor had he taken refuge with Paul, unless...are you thinking Roman citizens who were prisoners had input or authority on what to do with captured slaves?