Temporary VAT reduction by 3% until 31.12.2020

Are you sure that works? The flyer always aims at no additional tax increase being payable, but not at having the right to a subsequent tax reduction.

Example:
Advance payment November 2019 €15,000 + 19% VAT (explicitly stated)
Final invoice August 2020 €30,000 - €15,000 advance = €15,000 + 16% VAT to be paid.

Where does it say that I can offset or get back the 3% of the first €15,000?

I would say it depends on what has been contractually agreed with the general contractor.
If the service is only rendered upon final acceptance and thus the warranty period only starts then, in my opinion all payments made before are advance payments and therefore the VAT is only payable with the final invoice.

The clause in the property developer contract is as follows:

The clause in the developer contract reads as follows:

A developer contract is always without VAT, unless the developer opts for VAT when the recipient of the service is an entrepreneur!
§4 No. 9a USTG

In §13 USTG and §14(5) USTG.

Thank you very much. I really like §14(5)

At least this way I save a few euros on the idling of the craftsmen...

Does anyone have an idea of how to handle this with a larger trade that has individual invoices? Background: The heating installer has been working on the new build for about a year. I think we will complete everything with the commissioning of the heating in July. Partial invoices have been paid so far for parts already installed, etc. However, the final invoice will certainly fall into the exciting period starting in July. Can heating/plumbing be seen as an overall trade, where the warranty also only begins with commissioning and therefore the total amount is subject to 16 percent? I am clear on many issues, but I am torn on this trade. And the item is also relatively large, which could make it worthwhile.

Install the last sink in July and also conduct the heating instruction in July. Then both partial trades will be completed in July.