That is just dreadful service by that dealer. The only point in going to a dealer here is, because they have a large Mercedes sign outside. But it seems, they don't want to fix your car, they just want to get rid of you.
IMO, Find a good independant who wants your business and wants to spend the time to fix your car.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black