Pull the seat belt fully out, spray both sides and allow to retract, extend and retract several times, for good measure spray any plastic trim parts the belt may come in contact with, a good quality silicone spray should have no products which will stain or leave any smell.
2nd that heaps of silicone spray . just went through all my seat belts . The thing is The Mercedes l have owned would have to be the worst when it comes to sticky belts.
e220 coupe, Kawasaki ZX10R and a chugging H/D 883 Sportster.
If you have furniture polish in the house that isn't too smelly try that. My Impreza used to have "squeaky belts" and the polish cured that. It also made the car smell like it had been cleaned which was a bonus. Dave.