How does forgiving your neighbor because God forgave you relate to the difference between Law and Grace?
Because God forgave all your trespasses, you can do no less for your neighbor. That is the teaching, and it is identified as the whole difference between Law and Grace: God's forgiveness received becomes the reason and pattern for forgiving one's neighbor.
Les explains that since God forgave all your trespasses, the believer should ask whether he can do any less for his neighbor. He says that is the teaching, and this is the whole difference between Law and Grace. The context ties forgiveness received from God directly to the believer's willingness to forgive the neighbor.