“Now if he has defrauded you of anything or owes you anything, charge what he owes to me. I, Paul, have written this letter with my own hand: I will repay it. I could also mention that you owe me your very self.”
Paul doesn’t actually write what Onesimus has done, does he?
All he says is that if Onesimus has defrauded you of anything or owes you anything, charge what he owes to me. An offence has taken place, for sure, but Paul is willing to cover it all.
Maybe Onesimus stole from Philemon when he left, maybe his absence was costing Philemon money, we don’t know. What we do know is that Paul was willing to step in and have it [charged] to himself. Just pause and think: where have you seen this kind of offer before?
As Paul was willing to make substitutionary payment to ensure the forgiveness and acceptance of Onesimus, so Jesus was willing to do the same for you. Onesimus was in debt to Philemon, we are in debt to the consequences of our sin (Romans 6.23). Paul, here, took on the debt of Onesimus and on the cross Jesus took on yours (John 1.29, Hebrews 7.27).
Reading through Philemon, it would be all too easy to take a self-centred view: you need to forgive, you need to make things right with those you have wronged, you need to be like Onesimus/Philemon here.
This passage isn’t about you doing the forgiving and being oh-so-gracious, this passage is about the wonderful truth that God has been oh-so-gracious to you and has forgiven you through the finished work of Christ on the cross.
You don’t need to be Philemon here, the forgiver, because you are Onesimus here, the forgiven.