Adam Clarke on Philemon 1:19

I Paul have written it with mine own hand - It is likely that the whole of the letter was written by St. Paul himself, which was not his usual custom. See on 2Thes 3:17 (note). But by thus speaking he bound Philemon to do what he requested, as an act of common civility, if he could not feel a higher motive from what he had already urged.

Albeit I do not say to thee how thou owest unto me - I ask thee to do this thing to oblige me, though I will not say how much thou owest unto me; even thine own self, as having been the means of thy conversion.