The dealers are being unscrupulous ( is that spelt right ?? ) and the deposit is nothing to do with MB. If the MB dealer is using his direct and own production allocation then this should be cost free, if on the other hand he is " buying" another dealers allocation, this may be the cause of the varying deposits changing hands.
I personally paid £1500 deposit with nothing else until the car was in the showroom and checked over.
The danger is that, if the dealer for some reason is late with the car, they really have got you where it hurts.
Hope it turns out smooth for you .....