Putting the invoice aside for a sec, do you agree that there is a £740 short fall I assume they have shown you other documentation to prove this?
If you wanted to I suppose you could always make a good gesture to them and go halfs if you wanted to
Given you have been told and agreed the price of the extras and have the paper work, and that has made you decision to take the car, in my view there is nothing they can do.
Given your contract is with MB and not them, I would tell them to take it up MB as you have no contract with the dealer, and tell the dealer in your view you have met the terms of your lease contract, and fulfilled all payments needed as per your lease agreement terms official written documentation.
Ultimately unless this type of situation is covered in your legal agreement which I very much doubt it is, there is nothing they can do.
I would ask the dealer and mobility to point out in your lease agreement where a situation like this is covered, where a dealer can ask for more money when you have agreed the price, paid the price ,got your invoice, and car.
For good measure I would raise a complaint with MB.
If a dealer cannot be bothered to double check their finers, well tough look.
From a different angle if you was a private buyer, you went to look a car are agreed the price over the phone, paid that same price putting on the invoice, took the car away, and they triy and demand more money , haven’t got a leg to stand on