If you signed a contract with terms, and that contract had language such that it was impicite that the deal was based on securing financing, then you have to come up with the money. This is because many lenders will not make the loan if you are not at risk for a certain amount of the car...meaning 80% or so. That amount depends on your credit.
If now they say you have to come up with some money,and the loan was approved, and signed for, then this changes the terms of the contract, and you would normally have the benefit of saying, I do not like your terms of the new congtract, and give the car back.
But what is sticky here, is that I have no idea of how your loan arrangements went and how that was represented.
For example: some loans are not really approved, and the car agent mis-represents the lender by telling you it was approved...did you really see an approved loan?
This is a type of bate and switch operation, perhaps...and if it is, then contract is voidable.
This dealer is not likely to let you turn the car back...and so if you want to keep the car, then you must ensure your money is being used as a down payment.
For me, it is just a car...I will walk away from any car if I am being frauded....I would hire an attorney to deal with these guys. I have had clients who actuall ended up getting the car for nothing.
While that is not likely going to be your outcome, I think you stand to recover some cash, get them to stop, or void the contract so that you have a chance to walk away.