George Spiggott: You see, a soul's rather like your appendix: totally expendable.
George Spiggott: It's the standard contract. Gives you seven wishes in accordance with the mystic rules of life. Seven Days of the Week, Seven Deadly Sins, Seven Seas, Seven Brides for Seven Brothers.





Answer: It's a joke because the souls already belong to God. It's mentioned during the jail scene. So the question is irrelevant, as God already has well over 100 billion souls in any case, and the devil can't gain them back.
There is no jail scene in this movie. You're probably thinking of the remake.