V. -
I have a leased Chevy due to be returned in May of next year. My two year old unknowingly stepped in ivory paint and tracked all over my grey backseat when he got in the car! It was on his shoes, the backseat, the carpet, the back of the passenger's seat...
Anyway, I asked a few people what to do, and they suggested letting the dealership clean it. It's considered normal wear and tear, as another commenter suggested, and they don't penalize you for cleaning it. Besides, you'll come out of money trying to clean it yourself or having someone else clean it, and if (God forbid) your upholstery gets damaged further, then you'll most likely have to pay the dealership for having to refurbish or replace it. To be safe, just give them a call and double check.
Best of luck!