Reserve land before buying? - What would you do?

The farmer will simply have the problem that he has to pay taxes on the sale. And he cannot reinvest the money anywhere.

A registered notice of disposition (Auflassungsvormerkung) in the land register (883, 885 Building Code) prevents the sale to a third party.

So buying the property is your suggestion? How else should the notice of disposition be entered into the land register? I was rather thinking of a notarized purchase option, which is common.

Notary etc usually takes a few months to get an appointment anyway.

We went to the notary twice (once for the purchase contract, once for the mortgage deed) and each time it was within a few days (for the purchase contract, of course, plus the contract review period of one or two weeks or however long that is again).

Practically, you can already make the notarial contract now and secure the property for next year (July 2019). After all, the sale is not the date of the notary appointment, but the date named within it.

But honestly... when I read something like this

First of all, it is very difficult to have a reasonable conversation with him, as he constantly only gives vague statements (in principle, he would sell us the property, but to this day I do not know how much he actually wants for it)

... then I believe less that he knows he wants to sell.
He doesn't want to.

and secondly, his tax advisor advises him to wait until next year to sell, as this would be more tax advantageous for him (he is a farmer).

See above.

I see it like this. There is a farmer who has a piece of land. The interested party wants it, he has already approached the farmer. The farmer probably didn’t really want to... what should he do with money now, when there are no interest rates. Land can only become more valuable. So he is keeping the interested party at bay for now and watching what the market does. He will only sell, bets!, when he can buy a field elsewhere with the money, land for land. I know this, my wife is from the farm, that’s how the thinking goes there. Land counts, not money. Karsten