Temporary VAT reduction by 3% until 31.12.2020

I don’t understand this now. That was exactly the point.

As you wrote it, you have to carry out the acceptance of a trade; you have done this with the signature of the respective trade, which would make the matter clear.

The question remains what exactly is formulated in your contract. If you don’t provide these text passages here, we can only assume what you are writing here.

It is like always with such legal matters, if the "seeker of advice" is not practiced in subsumption, in the end, without the provision of the contract regarding the important passages, it results in guessing.

As you wrote it, you have to carry out the acceptance of a trade; you have done this with the signature of the respective trade, which would make the matter clear.

Where did you read that?

As you wrote it, you have to carry out the acceptance for a trade, which you have done with the signature of the respective trade, so the matter would be clear.

What remains questionable, however, is what exactly is formulated in your contract.
If you do not post these text passages here, we can only assume what you write here.

As always with such legal matters, if the "seeker of advice" is not practiced in subsuming, in the end, without contract input regarding the important passages, it becomes guesswork.

As the OP wrote, the acceptance took place after 07/01/2020, so the reduced rate of 16% should apply here clearly and without any discussion!

Well, at least I think we all agree that without taxes everything would be easier

As the OP wrote, the acceptance took place after 01.07.2020, so the reduced rate of 16% should clearly apply here without any discussion!

How do you know when the acceptance between the main contractor and subcontractor took place? The OP may be the invoice recipient in this structure but possibly not even a contracting party, because the contract was only concluded with the main contractor.

Well, at least I think we all agree that everything would be easier without taxes

Value added tax alone accounts for around 25% of total tax revenue. This would likely cause state and municipal budgets to falter significantly.