Price adjustment clause in the contract with the general contractor

The wording "If the material price and thus the craftsman services change..." would make me suspicious. The craftsman service remains exactly the same - whether the roof batten costs €4 or €14. That would actually mean that the price could be arbitrarily adjusted here. Apart from the fact that, from my point of view, this would also fall apart in court, one might perhaps phrase the passage so that a percentage increase in material costs can be passed on accordingly. With a trustworthy general contractor, this would be a fair arrangement that would distribute the risks mentioned by 11ant accordingly. That in turn would of course mean that detailed proof can be provided by the general contractor.