Temporary VAT reduction by 3% until 31.12.2020

see paragraph 1 Preisangabenverordnung
According to this, the gross price must generally be stated.
Whether there is another civil law regulation somewhere, I cannot say.

see Paragraph 1 Price Indication Regulation
According to this, the gross price must generally be indicated.
Whether there is any other regulation under civil law, I cannot say.

But isn't the gross price and VAT always specified (at least I have never seen otherwise)? I have never seen an invoice = €119 gross. Actually at least "€119 incl. 19% VAT" or "€100 plus €19 VAT".
To my knowledge, there is an obligation to show the VAT.
It is different if I buy a plot of land and the house to be built is included. Then no VAT is due for the house because the land and house are taxed with real estate transfer tax.

No VAT is to be shown in the offer, or do you see it on the product in the supermarket? There, the gross amount must always be stated. In contrast, the net amount is also shown at Metro. Whether there is a change to the amount when the net amount and VAT are also shown is a civil law question and not a tax issue. However, I am not familiar with that. All tax articles I have read on this matter refrain from answering the question and refer to civil law.

see Paragraph 1 Price Indication Regulation There it is generally required to state the gross price. Whether there is another civil law regulation somewhere, I cannot say.

However, this by no means implies that the general contractor can pocket the 3% VAT. In the final invoice, the gross price is simply adjusted to the new tax. I cannot imagine the general contractor making a profit from the VAT reduction even from an accounting perspective.

No VAT is to be shown in the offer, or do you see it on the product in the supermarket? There, only the gross amount must always be stated. In contrast, in Metro the net amount is also shown. Whether there is a change to the amount when the net amount and VAT are shown in addition is a civil law question and not a tax issue. However, I am not familiar with that. All tax articles I have read on the topic refrain from the question and refer to civil law.

Well, in retail a contract is concluded at the checkout. If an offer is accepted there (that is, by the buyer), a purchase contract arises, and only then, at the checkout. The invoice then issued (the receipt) does indeed contain VAT. The price tag in the supermarket is only an offer. Nothing more. The situation in the supermarket is somewhat different because it is a spontaneous purchase.

But that by no means means that the main contractor can just absorb the 3% VAT. In the final invoice, the gross price is simply adjusted to the new tax rate. I cannot imagine a profit for the main contractor from the VAT reduction even from an accounting perspective.

Put yourself in the entrepreneur's shoes:
A gross price was agreed with the end customer. According to the Price Indication Ordinance, this must always be gross.
Why should the gross price now be changed? Which law obliges him to adjust the price?
In my opinion, theoretically even the individual prices for the end customer should be stated or at least be able to be stated gross:

Earthworks €50,000
Shell construction €100,000
Sanitary €25,000
Electrical €25,000
Other €50,000
Gross €250,000
including VAT 16% €34,482.75

of which €238,000 was prepaid
still to pay €12,000

done.

Now imagine the reverse situation. You agree a fixed price of €250k with the entrepreneur.
Now the VAT is spontaneously increased and you suddenly have to pay 3% more. Would that be fair to you or would you sue for your fixed price?
That is exactly why most main contractors will have a provision in their contracts that entitles them to charge the increase on top.
They have learned from historical VAT increases that such a change in VAT leads to lawsuits.

However, if there is no provision in the contract, none of you have so far been able to name a legal basis for me (except the §29 USTG I mentioned for long-term contracts) that could force the entrepreneur to deviate from his agreed gross price, except for his own goodwill.