The Great Exchange
From Mark Horne’s blog:
The great exchange means you are dealing with Jesus in that Christian who sinned against you.
So if you consider me your partner, receive him as you would receive me. If he has wronged you at all, or owes you anything, charge that to my account. I, Paul, write this with my own hand: I will repay it—to say nothing of your owing me even your own self.
So writes Paul to Philemon about his runaway slave, Onesimus.
Notice the exchange that takes place.