Contract clause for subsequent purchase price adjustment in the new development area

Exactly. And after 21 years then actively fight for more building land.. ;)

Blocking for 20 years shouldn't be a problem.

Does this obligation have to be passed on in case of resale? Or does it only apply to you as the buyer?

Otherwise, you can establish a Mini GmbH and sell the meadow to it or alternatively sell it to a woman, child, brother, etc. In the case of a meadow, the real estate transfer tax cannot be too high. The meadow does not factor into your floor area ratio and setback regulations. What if you place the house directly on the border to the meadow? Then a fictional buyer would have to comply with the setbacks for you; possibly, it would hardly be usable as a building plot, and the price per square meter would be correspondingly lower.

I recommend confronting a lawyer with such ideas; they might find a legal leverage here.

No, so buying a meadow and selling it right away unfortunately doesn’t work, then a speculative tax is due and that one is really quite high...

Moreover, as far as I remember, it is also the case that you cannot sell the meadow for < 10 years without paying this due tax. That means, if you want to sell it again beforehand because you cannot afford this "quasi back payment," a very expensive amount, namely this tax, is due anyway.

Speculation tax requires a profit. If there is none, there is no tax. However, I hardly believe that the notary was so sloppy as not to apply the obligations in case of resale as well. These are really standard clauses in the contract. One should rather think about which municipality would have an interest in converting garden land into building land if its owners mostly signal that they do not want to sell anyway.

Ok, may be. I would still be cautious whether that is so easily possible. Assuming that the OP only wants to sell if he has to pay extra, there would be a profit.