If you overpaid to your baloon payment, you could presumably either sit for 2 more years paying interest on the balloon payment balance owing every month (think of it like an interest only mortgage), or get a loan out for the baloon payment, clear the Audi finance and start paying off the new loan.
I also believe that Audi do PCH finance - you basically pay off the whole balance over 5 or 6 years (less if you overpay).