Down payment on a car & then he "changed his mind"?

So my husband got me a firebird for vday off of CL. He gave the guy a down payment, and supposed to give him the rest of the money Fri. The guy says his daughter now wants the car, even after he wrote up & signed something. The problem is this AFTER my husband hd "purchased" the car the guy fixes his ad and adds pics! My husband thinks someone offered him more money, which i think hes right. This was supposed to be a suprise for me, feel bad for my husband!is this legal,?? this cant be alright! thanks for any input!