Note that Paul "Requested" that Philemon release Onesimus as a Brother, as he stated he could be of much service to him (Paul). He DID NOT Command him to do so & he did not tell him it would be a Sin not to. One has to read what the Bible Says, Not what one wants it to say. The freeing of Onesimus was purely a personal request from Paul, not a Command of God.
1Co 7:21 Art thou called being a servant? care not for it: but if thou mayest be made free, use it rather.
1Co 7:22 For he that is called in the Lord, being a servant, is the Lord's freeman: likewise also he that is called, being free, is Christ's servant.
1Co 7:23 Ye are bought with a price; be not ye the servants of men.
1Co 7:24 Brethren, let every man, wherein he is called, therein abide with God.
From a Greek Lexicon, the word Servant here is literally "SLAVE".
Nota Bene (Note Well) I am not suggesting we return to slavery. I am quite happy it no longer exists. I had ancestors who owned slaves. Some or all of them may well have been sinners in the sight of God, he will Judge that. I will state with Confidence though if they were, it was NOT because they owned a slave or slaves.
There were laws given, from God, regarding one's treatment of their slaves. It was never a command they had to free them. Also, note I am not taking it upon myself to "Speak For God" I am simply letting "His Word" speak for itself.
My point is one simply cannot Judge the past by the present. Abraham is referred to as The Friend of God & also as the Father of the Faithful. He owned many slaves, he was never told to free them. He was given many commands, which he obeyed, but that was not among them.