Can car dealer add stipulations after signing?

i bought used car from new mexico dealership put 1,000 down and they gave me one thousand dollars for a trade in. i signed a lot of papers saying the deal was complete, and then today, six days after the deal they called and are now asking me to go in and talk and are asking for a cosigner. i already have the new car i got from them and signed papers to complete the trade can they add more conditions when i go to talk to them even though i already signed the papers and traded in my car