This doesn't make sense. If the sin is "already forgiven," there can be no punishment to remove! That's the very definition of forgiveness. The idea of What you have done here is put a "spin" on it in order to defend this evil and corrupt practice. And you say nothing about the exchange of MONEY(!) for these indulgences. In Luther's day, they were sold for money!! Peter said, "and your money perish with you" to the man who would purchase his way to heaven.
Furthermore, if "good deeds" could gain forgiveness, then there would be no need for Jesus Christ to die on that cross. It was the ONLY WAY! If there was another way, do you think God would have sent His Son? His death was the ONLY WAY that God could wipe away sin. It is blasphemous to suggest even remotely that there is any other way.