Thank you very much for the contributions.
A brief clarification:
I don’t earn badly, but €2,100 is not “peanuts” for me, sorry. That’s why I put brainpower into this. €2,100 is not an amount I just take from petty cash without noticing.
According to the tax advisor, it is possible to separate "gift" and "exchange." And I believe her; she has no personal interest, as she is a family friend who supports us like this. I know her as a very experienced and good tax advisor with her own firm. Therefore, I trust what she tells me (she also fought through various legal interpretations especially for this).
A sale below value is tax-wise treated like a "mixed gift," and as already mentioned here: the difference between the lower purchase price and the real purchase price is declared as a gift and taxed accordingly. The tax office is not that stupid; otherwise, everyone would sell below price to avoid taxes.
We don’t want to “cheat” either but only use the existing legal situation in the variant most favorable to us. Unfortunately, we did not get any advice from the notary in this regard. That is to say: “You can do it this way or that way. The advantage here is xy, disadvantage xx. The advantage of the second variant is ab, disadvantage cd. Now you have to decide what is more important to you.” That’s what I had imagined, but unfortunately it was not like that. We left the office and basically had the promise that it would be implemented as discussed (gift and exchange/sale – we presented the proposal to the notary that we had worked out with the tax advisor) only to now hold a contract draft in hand in which this was not implemented accordingly. And with the indefinite feeling that the notary just wanted to make things easier for himself, which only causes us costs. He gets the same anyway.