Press release button 4 and guide belt tongue 2 back towards belt sash guide 1.

Make sure that the seat belt is fully rolled up. Otherwise, the seat belt or belt tongue will be trapped in the door or in the seat mechanism. This could damage the door, the door trim panel and the seat belt.
Damaged seat belts can no longer fulfill their protective function and must be replaced. Consult an authorized Mercedes-Benz Center.