Am I getting scammed here?

[Ringo]

I recently financed a preowned '05 chevy colorado at an out-of-state dealership about a month ago. I am from NewJersey and the dealership is in Maryland. Anyhow I just got a phone call from the salesman yesterday(1 month after the sale) and he tells me that they made some kind of mistake with the state taxes during the sale and that I owe them $300 to clear it up in order for them to send the title work to my state so I can get the truck registered. So basically they are telling me I wont be able to get it registered unless I pay them the difference. I asked him how he messed this up and he told me that the county taxes are changing all the time and this happens every now and then. I was busy at the time so I told him id have to call him back later to clear this up. I am very upset that im being "back charged" here but dont know what to do. Has anyone ever had something like this happen to them or does this sound like a scam? Any advice on what I should do here would be much appreciated. Thanks