You got screwed by the stealership...
First of all, you never should trust a car dealership on their 'word'. Probably everyone on here has been burned or been misled by one dealership or another. Unless you have something in writing by them, never trust what they say or agree to.
I doubt your used wheel spacers and exhaust - $300 are worth the amount of time and money you would have to spend on legal counsel to handle this issue. I would just walk away and take it as a lesson learned.
It totally sucks, but if it makes you feel any better... I can personally say you are not the only one who has been screwed by a dealership. I would take the $300 and forget about it. If you can afford to trade in your 2011 M3 for a newer vehicle, it sounds like you can afford to charge this one to the game.
Jet Black E92 ///M3 | Bone Stock