May I suggest the next time you buy a car, Get up from the dealers table and start to walk away.
I bet the deal gets better not worse.
Your guarantee is worth the paper it is written on.
Last time I was involved in buying a car, the dealer made an offer which we considered to be too high.
We walked away.
A short while later, dealer made another offer. Higher. (From fallible memory, I think we rang to ask if the offer was still the same in the hope of getting an improvement.)
We walked away.
We then noticed a substantial change to the "scrappage" allowance and went back. Dealer factored that in (plus a couple of other improved "inducements") and came up with an acceptable deal.
It wasn't the walking away that got a better deal, it was that the offered deal was over our self-imposed limit. Then there was a policy change completely unrelated to our walking away.