He paid a debt he did not owe
I owed a debt I could not pay
I needed someone to wash my sins away
So now I sing a brand new song,
Amazing Grace, because he paid a debt
That I could never pay.
What I didn’t know what that it was lifted from Anselm:
Nor did anyone ever pay a debt to God which he did not owe. But Christ of his own accord gave to his Father what he was never going to lose as a matter of necessity, and he paid, on behalf of sinners, a debt which he did not owe. (Why God Became Man 2.18, in Major Works, p.349)
Of course, I’m stretching a little. But the idea’s there.