The dealer made the mistake. If anything, the finance guy should have checked the numbers before having you sign it. Legally, what you signed is legit and they cannot come after you. However, it will be a goodwill gesture for you to go back to sign the revised contract. It's like saying "sorry I charged you less, can you come back to pay the difference".
2013 BMW X5 35i
Alpine White | Cinnamon Brown | M Sport
2013 BMW 550i
Titanium Silver | Black | M Sport