Id take them for the full value of the car as it was!
Fucking wankers!
Last time I took my car to a garage for some work he said are you leaving it with us? No Ill wait with the car thanks.
He looked really gutted and tried to talk me into coming back for it but I knew his ideas....
I ALWAYS wait with the car now, its the only way to be certain they are not going to do a Ferris Beuler up the road soon as your back is turned