$9000.00 is a pretty big screw up, don't know how they missed that and I'm sure some employee is praying to keep his job. I've had my employees under- charge the customer on accident. In the past if it's been over a $100.00 I've called the customer to explain the situation and then the balls in their court. Most understand but some don't, thankfully it's only happened a few times.
Sounds to me like the deal is done, signed and sealed... I'm guessing if it was a screw up in their favor you would have been told that you shouldn't have signed if you didn't agree. Think I would keep it locked and blocked in until you get this all taken care of.