If they do that, the only thing to say is "good bye, and the 'good' ia there for politeness' sake only". Never ever talk to them again and if you have time, write most nasty review on them on any site that takes reviews (of course, tell the truth, but don't be shy about it). This would be good both for you and for the environment - sleazeballs should not be rewarded.
Fortunately, my experience shows there are honest dealers that do not renege on their promises and respect both themselves and their customer. Even in this business, notorious for some bad apples, there are also decent ones.
