GMAC Smart Buy Car Loan

We purchased a car under a GMAC Smart Buy - lease with option to purchase with a balloon payment at the end of the contract. We could not afford the payments and tried multiple times with GMAC to refinance the car and they would not work with us. We turned the car in @ our local dealership. GMAC contacted us saying we owe the balloon of $27k on the car - even though we turned it in. They have now sold our debt to a collections agency.

Since this was a lease with the option to buy and we turned the car in do we owe the balloon payment? Now that they have sold our debt - what obligation do we have to the collection agency? Our credit shows a charge off from GMAC.