A GM salesperson can not sell you a vehicle that his dealership doesn't own (or doesn't have the ability to own). The only way that he could possibly sell you a Mazda is if his dealer group has a Mazda store. However, make sure you check out the Kia Forte before you buy a Mazda3.
Once they sell it (you can't sell it) at auction for auction value and add on the fees involved they will try to collect the difference from you. You can either make payment arrangements to pay the balance or you can avoid it until they sue you and obtain a judgment and garnish your wages or you...