Yes, exactly that is stated in the contract, e.g. 10% of the total amount after the installation of the windows.
10% is not a concrete partial service but a deduction.
It depends on the
acceptance, i.e. when the risk transfers to you.
If no specific interim acceptances are agreed upon in the contract, then the final acceptance is decisive.
If interim acceptances were agreed, the risk would already transfer to you and the warranty periods would also start.
However, this is unusual with a general contractor.
If you finish in September, then the total service has to be taxed at 16%.
The only question is what the contract states regarding who gets the 3%.