Something is real fishy about this story ...
Dealer had only one used M3 on the lot (I assume it is the OP M3 that they took back from him)
Try to contact the dealership about this 2008 M3 and they will come back to you and tell you it was sold last night.
Sorry OP seams they took your $20K and sold your repossessed car again last night.
Oh it is still listed, they do explain it will take a few days to remove a sold car from there web site inventory.
The car in question (2008 M3) was $44K not $50K
Perhaps the OP put some score marks on the car ? and devaluated it since driving it of the lot ?