I had similar marks on my C320 driver seat when I first bought it. I took it to the dealer's. They tried to remove the mark with 3M Adhesive Cleaner. It got a little of it up, but not much. Finally, they gave up and sprayed leather dye on the area (I have ash colored leather). It worked like a charm. The dye completely covered up the stain and you can't even see a trace of it now. You could go to the dealer and ask them to do this for you, or buy the dye yourself. Also, I found a website
http://www.leatherrepairkits.com/ that sells leather dyes mixed to match most car interiors (including MBs).