Land expected for development, who sets the boundary here?

That the area will ever become building land is as uncertain as playing the lottery

I am selling to a local professional developer. He will know what he’s doing. ;) Maybe he also likes playing the lottery :)

That’s why the question: Why should anyone pay you more there?

No, it’s not about being paid more... It’s about selling more. If I get rid of more of the land, in the end I also have more money.

But to be honest, that’s not as important as the question whether, and if so which lease plots from the allotment garden are affected by the sale/the new boundary.
Because that, in turn, determines whether and how much possible compensation payments have to be made to tenants of the association.

I assume, before you draw up a purchase contract, the property would first have to be divided into two parcels. Probably a survey would take place at the same time?

No, the exact survey and registration takes place afterwards. First, an estimate of the square meter size is made and paid accordingly. If it turns out that the area is smaller, I pay back something, and vice versa.

Alternatively, maybe the prospective buyer is also interested in the entire property including the allotment garden?

No, he only wants to buy the land that he can develop into building land and then sell profitably.

Okay, I will try to reach someone at the city...
Or a local surveyor. They should also know how this process works.

I can't make sense of the two pictures.

The two graphics are not related to each other. That means the first sketch I created as a general representation of the situation (an area with two uses and somewhere a future boundary). The second illustration is from the land use plan of the city. Imagine that the area with the W7 symbol and also the hatched one with "0.5" belong to me (one parcel). Now the dashed boundary line of the agricultural land is so thick that it is far too inaccurate for me, and I need to know exactly where this line runs within a few meters. Do you understand it better now?

Is there no map from the Geoportal where the 20,000 sqm can be seen? In the Geoportal, you can even access the different maps. Then you lay the property boundaries over the land use plan.

Which Geoportal exactly are you referring to? I just looked at it in the Sachsen-Anhalt Viewer. I can get a map from the cadastre there, but no map of the land use plan. Or am I just too dumb? What is it called in the map selection on the left?

Just ask the local planner who created the plan. Hopefully, they can tell you the dimensions. Because they also need a length x width specification for the size of the area.

You do not understand the illustrations you have shown - and we can only help you very limitedly, since you thwart the overall overview by 1. not including the representation from your other thread here, and 2. also not consistently applying the scale of the illustrations. The thick dashed line encloses an area of applicability - here it backfires that you do not depict the entire area. W will stand for residential development, and W4 and W7 presumably for different permitted uses, 0.5 will presumably be the targeted maximum floor area ratio, and green is supposed to remain grassland; yellow are traffic routes. Overall, I think you are hearing the money grow years (if not decades) too early. Be patient longer than you would like :-)

the overall overview

At least approximately, one can recognize in your old thread in post #11 (and in my impression, basically everything essential has already been told to you there - partly by experts who are no longer present here).

I don't know how it works in the East of Germany, but here in the South the municipality buys the land, develops it, and then sells it. This is meant to prevent speculation. The decision on how big and what is built is made by the municipal council. Bauerwartungsland has no legal significance. Make a preliminary building inquiry, then you will know how big and what the chances are.