Don't try to sell the diesel. It's extremely nasty, minimise how much you handle it. If it wasn't in such common use we likely wouldn't allow it to be handled the way it is.
Contact Mercedes. They are likely to advise you of the best situation. It may be possible to continue using your vehicle with this fuel, just don't use this fuel regularly (ideally never again). The reason it's advised against may just be fuel efficiency or emissions standards. It may not harm the vehicle. It also may harm the vehicle.
If you have to siphon the fuel tank, please dispose of it properly. Your local authority should have a recycling centre/waste disposal where they can dispose of this often free for personal use. Don't dispose of it in your garden or drains. It's bad for wild life, children and you.
It's often very difficult to siphon a newer vehicle, they have anti siphon features. If your not comfortable fixing vehicles, it may be best to get a mechanic to do this for you.