Adam Clarke on Philemon 1:14

That thy benefit should not be as it were of necessity - If the apostle had kept Onesimus in his service, and written to Philemon to forgive him and permit him to stay, to this it is probable he would have agreed; but the benefit thus conceded might have lost much of its real worth by the consideration that, had he been at Colosse, Philemon would not have sent him to Rome; but, being there and in the apostle's service, he could not with propriety order him home: thus the benefit to the apostle would have appeared to have been of necessity. The apostle, therefore, by sending him back again, gave Philemon the opportunity to do all as if self-moved to it. This is a very delicate touch.