4 years after construction, the office also demands land acquisition tax on the house

You can find the connection in the Real Estate Transfer Tax Act §8. In your case, the purchase of the property is tied to a pre-determined general contractor.
Most recently, in 2022, the Federal Fiscal Court already regarded a much looser connection as a tied transaction (BFH decision from 7.2.2022, II B 6/21).

Quotation for explanation: "The current decision of the BFH shows that even without - simultaneous - written agreements between the contracting parties, a unified contract structure and thus the extremely high real estate transfer tax can arise. To achieve a separate treatment of land and building and consequently incur real estate transfer tax only on the land, you must take care of an appropriate construction company yourself, which has no relation whatsoever to the seller of the land. The emphasis is on the word 'no relation whatsoever.'"

And you are very far from 'no relation whatsoever.'

There are dozens of rulings on this - unfortunately all in favor of the tax office. Sorry...

Do you also have the source for the text in the lower half of your post?

[*]the linkage in the contract text,
[*]the consolidation of the agreements in one document, or
[*]an overall price.

Since the seller of the land wanted to build a semi-detached house, the purchase contract for the land at that time stated that we commit to constructing a semi-detached house with a specified general contractor (who has no further connection with the seller of the land).

Nothing has yet been determined regarding the house. There was a demand for disclosure of the invoices for the determination of the tax. Could I now delay this beyond the turn of the year?

As an alternative to the explanation, Haufe is also quite good because it is understandable for non-lawyers – see the red parts:
Legal Context
If the parties make independent agreements about the obligation to transfer the property and about the provision of construction services, the decisive factor for the assessment is whether, from the overall circumstances, the will of the contracting parties emerges that the object of the acquisition transaction is to be the developed property. Indicators of such a will of the contracting parties are, for example

[*]the linking in the contract text,
[*]the consolidation of the agreements in one deed or
[*]a total price.

In such a case, the object of the acquisition transaction is the developed property (cf. BFH ruling of 13.04.1983 - II R 53/81).

If there are two or more contracts, they are to be considered as a unit if they are explicitly dependent on each other. This also applies without an explicit link of existence if, according to the will of the parties, they are so interdependent that they must "stand or fall" together. Whether this is the case is to be determined by taking into account the interests of the contracting parties, their behavior before and at the conclusion of the contract, and the actual course of events (BFH rulings of 13.08.2003 - II R 52/01 and of 21.09.2005 - II R 49/04).

Objectively Close Factual Connection
An objectively factual connection between the land purchase contract and further agreements leading to the assumption of a unified acquisition object exists, among other things, if at the time of concluding the land purchase contract the buyer was no longer free in his decision regarding the “if” and “how” of the construction measure in relation to the seller and it was therefore established that he would only receive the property in a certain (developed) condition. This can result from a certain chronological sequence of the contracts or from a factual compulsion (cf. BFH ruling of 30.08.2017 - II R 48/15, BStBl 2018 II p. 24 with further references).

Prepared Course of Events
Furthermore, an objectively close factual connection may exist if a prepared course of events is accepted. A prepared course of events is to be assumed if the seller of the property offers the interested party, based on a concrete and approximately construction-ready preliminary planning, certain construction services on a specific property at a price that is essentially fixed and the interested party can only accept or reject this offer as a whole. For this assessment, it is irrelevant whether the preliminary planning was substantially influenced or even initiated by the buyer’s side (cf. BFH ruling of 21.09.2005 - II R 49/04) or whether the offer is accepted with minor deviations (cf. BFH ruling of 03.03.2015 - II R 9/14).