My face in exchange for yours?  
Your mind for mine?
How about body?
Flaws?
Mistakes?
Spirit?
Heart?
The only catch is that you give me all of your bad deeds and I keep mine.  But you must accept all of my good and stockpile it with the infinate abundance of your own.
Maintain this state until judgement.
Sound good?
Sincerely,
The Sin Eater