Can the car dealer really make me do this, help?

I brought a new car this weekend & the dealer told me the credit had went through w/my mom being the co-signer. They called me just now & told me it didnt go through & that the car would have to be in my moms name ONLY.They said if my mom does not come in & sign that I will have to return the car. Can they really let me drive off the lot with the car & then have to return it? I already put 350 miles on it.