It's on the owner to prove at the dealership broke something. Very difficult to do. When I had my Toyota Yaris, I had the Takata airbags replaced. When the Toyota mechanics remove the dash and replaced it they broke driver side cup holder. It would never retract and stay closed. I couldn't prove it wasn't that way when I brought the car in.
It was on me to prove that the dealership broke it. I'm certainly not going to get a lawyer for something like. I don't think that most people would. The repair is less than what lawyer would charge.
Almost an impossible position to be in.
While I agree that an attorney is a bit of overkill, the question remains, was the air bag light on before the owner brought the vehicle in for the seat belt swap?
If the light was
on... any reputable
dealership or repair shop would make note of that with the customer before even touching a vehicle for any
type of repair.
Problem is on the vehicle owner.
If the light wasn't
on before the work, and there is no record of it being on... then the vehicle should be returned in the same condition.
Problem is on the shop.
Just telling the guy he needs to spend another $1000, and also stating, per the OP, "they don't know what's wrong," is not what I would expect from a Mercedes service department.
I've been managing a towing company for over 20 years, and if I had a dollar for every customer that said, "well it worked before you towed it..." I would retire wealthy.
I fully understand, proving fault, but there must be more to this particular story, surely.
[In re-reading the OP's original post, he states, "light is still
on," which now makes me believe that the light may have been on before they replaced the seatbelts... which doesn't look good for him. But he still hasn't said why
the seatbelts were replaced to begin with. Need more info here.]