Contract for work before property purchase - real estate acquisition tax?

We found the plot on the internet; it is marketed by a company specialized in land development. There is no developer obligation or similar.

Completely independent of this, we signed a contract for work with a construction company a few weeks ago. At that time, we did not know the building area yet; we originally had a different building area in mind.

So both companies are in no way connected.

Hello everyone,

Unfortunately, the post went live faster than I could type it, which is why the subject unfortunately only contains a comma instead of "Real estate transfer tax on contract work agreement BEFORE property purchase."

Regarding my concern:
We concluded a contract work agreement with a renowned house manufacturer BEFORE we had a piece of land.
Now we have a suitable plot of land in a new development area in sight, but the land seller said that we will very likely also have to pay real estate transfer tax on the house. The reason given was that we concluded the contract work agreement before purchasing the land.

Could that be true?
The land seller and house manufacturer are completely independent from each other; they don't even know each other.

Best regards