Yeah Zate sorry but let me play devils advocate here. Everyone wants dealers to be open on evenings/nights and weekends so it's convient for them to shop for a car. Finance companies are open M-F 8-5 EST in most cases if not all. If you buy a car after these hours the dealer has to use the judgment of the in house finance guy. He apprently thought your credit was good enough to qualify, not his fault the actual finance agency turned you down.
Did it not seem odd when you were signing the paperwork that the finance information was not documented clearly? On my it clearly stated Chase was the finance institution and the intrest rate was blah and the payments were xxx.xx. They must have had you sign off something that said the fiance was not locked in and pending their approval. If you were questioning your own credit why take a car before you know for 100% sure your qualified.
I just did this with my 01 Cobra less then a year ago. I went in had them get me approved in advance and the intrest rate I wanted before I even selected the car. There was no question at that point. The car buying experience is just that, experience. The more you do it the more you know what to do and when.
This was not really directed at just you. I've heard this story so many times before and everytime it's the dealers fault never the fault of the consumer. Sorry to sound so blunt but it's the consumers job to be a educated buyer.
Good luck with your future automotive buying experiences, hopefully you at least walk away knowing alot more about the process then you did previously. That alone will help you tremedously next time around.