I recently had a similar situation where the dealer included everything on the invoice, I queried it, they checked it, said it was right, I said good and paid it. Then they didn’t get as much as they thought from Motability and their accounts department raised it with sales. Sales phoned me asking for more money, I reminded them of what I’ve just said here and that no, I’d paid the full invoice, which in fact had been presented and confirmed before, and they would not be getting any more money from me, if they’d made a mistake it was down to them. End of.