I recently purchased a car and the contract was signed, now they're telling me I'm not approved! Help?

I just bought a car on Saturday, May 14, 2011. I traded in my car and paid an additional $950. The contracts were signed by both myself and the dealership and I drove the car home that day. Four days later, on May 18, I got a call from the salesman that I worked with, telling me that there is a problem with approving the credit amount without a co-signer. Now if that was the case why was the contract drawn-up and signed by both parties? Doesn't the dealership have to have the loan approved first before the contract is signed? I later received a call the following day, from the same guy, saying that I can be approved for the car without a co-signer only if I put down an additional $3,000.00 or add another $30 to my monthly payments. I did call a lawyer that deals specifically with claims against car dealerships and left a message. I haven't got a returned call yet. Can someone please help? There is more to this story, but I don't have enough room in this box that will let me type it all out. What I stated is just a portion of the problem...