Philemon had a fair claim over Onesimus, and probably wasn't a bad master. Paul seems to have confidence in him as a believer. However, Paul isn't asking Philemon to do something within his rights, but to consider Onesimus as a fellow brother. This would have been somewhat radical, and would have required a sacrifice on the part of the owner. We don't have an answer from Philemon to Paul's challenge, but if Paul was a good judge of character, then Philemon may have indeed made good on this.