Albert Barnes on Philemon 1:19

I Paul have written it with mine own hand. It has been inferred from this, that Paul wrote this entire epistle with his own hand, though this was contrary to his usual practice. Comp. Rom 16:22; 1Cor 16:21; Gall 6:11. He undoubtedly meant to refer to this as a mark of special favour towards Philemon, and as furnishing security that he would certainly be bound for what he had promised.

I will repay it. I will be security for it. It is not probable that Paul supposed that Philemon would rigidly exact it from him, but if he did, he would feel himself bound to pay it.

Albeit I do not say to thee how thou owest unto me thine own self besides. Paul had doubtless been the means of the conversion of Philemon, and whatever hope he cherished of eternal life, was to be traced to his instrumentality. Paul says that this was equivalent to his owing himself to him. It is very life--his eternal welfare---was to be traced to his labours. What he asked now of him was a small matter compared with this, and he seems to have supposed--what was probably true--that for this consideration, Philemon would not think of exacting of him what he had voluntarily obligated himself to obey.

(*) "do not say" "not to say"