Go talk to them, I'm sure the manager will straighten you out. They will have receipts and records for the day and they'll be able to find the error. The amount that goes through is manually punched in so the amount charged will not match the receipts.

And use a credit card. One call to AMEX is all it takes and they'll go after such charges. Its a blessing - kinda like having my own hitman I've had to use them to help me out with several such incidents and they're awesome.