Temporary VAT reduction by 3% until 31.12.2020

Why shouldn't there be 19% on that?

The question is when the VAT is really due. Clearly with the final invoice.. but with a down payment before even one handshake has been made, I no longer find that so clear.

That should be the case, because the money flows then as well.

The decisive factor will be the date of service provision, which was also the case with the increase in 2007.

Flyer of the Verbraucherzentrale on Google: flyer_mwst_2006.pdf

The decisive factor will be the date of service provision, which was already the case with the increase in 2007.

Flyer of the Verbraucherzentrale on Google: flyer_mwst_2006.pdf

Exactly. The partial payments according to the installment/payment plan only have the character of advance payments on the agreed total price.
Therefore, the rule is probably quite simple: Date of final acceptance = Date of service provision

Ergo, one should try to arrange the final acceptance by 31.12.2020.

Example:
- Completion and final acceptance on July 15, 2020
- Total construction costs: 450,000 EUR gross (including 19% VAT) according to the construction contract; of which 400,000 EUR has already been paid in installments

Final invoice: 450,000 EUR / 1.19 * 1.16 = 438,700 EUR
Payable with final invoice: 38,700 EUR
Savings or indirect subsidy: 11,300 EUR

Sure that this will work? The flyer always aims to ensure that no subsequent tax increase has to be paid, but not that you have 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% from the first €15,000?

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

In §13 USTG and §14(5) USTG.

If the entrepreneur receives the remuneration or part of the remuneration for a delivery or other service not yet performed, paragraphs 1 to 4 shall apply accordingly. If a final invoice is issued, the partial payments received before the performance of the delivery or other service and the tax amounts attributable to them must be deducted if invoices within the meaning of paragraphs 1 to 4 have been issued for the partial payments.