I have the same problem. Seems to be caused by belt loops abrading the leather when you get in and out.
There are at least 2 companies which market leather dye to the consumer. Check out Leatherique.com and colorplus.com. I have used both with good success. The advantage of colorplus is they stock MB standard upholstery colors. Their gray was a perfect match for my leather seats. It's pretty straightforward to apply-- comes with good instructions. Like painting your house, preparation is everything.