John Calvin Commentary Philemon 1:14

"but without thy mind I would do nothing; that thy goodness should not be as of necessity, but of free will." — Philemon 1:14 (ASV)

That your benefit might not be by constraint. This is drawn from the general rule that no sacrifices are acceptable to God but those which are freely offered. Paul speaks of almsgiving in the same manner (2 Corinthians 9:7). Τό ἀγαθον is here used for “acts of kindness,” and willingness is contrasted with constraint when there is no other opportunity of testing a generous and cheerful act of the will; for that duty which is generously performed, and not through influence exercised by others, is alone entitled to full praise.

It is also worthy of observation that Paul, while he acknowledges that Onesimus was to blame in the past, affirms that he has changed. And so that Philemon would have no doubt that his slave returns to him with a new disposition and different conduct, Paul says that he has thoroughly tested his repentance through personal knowledge.