what should I do?

I sold a car for 2000.  They wanted to do payments they gave me 400  And were going to give me 400 every two weeks. But they called me and said they took the car to the  mechanic and said they were going to have to put in 500 To fix the water filter so we told them well do 1500 cause We didn't know something was wrong with the car since It has been parked for quite awhile.and now there calling To say either we give them the title..or to give them the 400 back and take the car..and I don't know what to do Cause I don't have the 400..please help.