You are writing about a new development area, purchase negotiations, and a farmer who previously owned the land. From this, based on experience, I read three things:
New development area.
This sounds like an area consisting of building land that has only recently been designated as such. Nowadays, this usually means that a development plan has been or is being drawn up. The drafting process or the legally binding development plan is no secret, regularly the subject of public council meetings, and nowadays also commonly mentioned on the municipality's website.
Farmer.
Previous agricultural use suggests that the parcels must first go through a land consolidation procedure before development. Fields are often long to minimize tractor turning, but for houses, more width is preferred. The areas for roads and accompanying greenery must also come from somewhere. Typically, all parcels are put into a "dice cup," thirty percent is deducted for public areas, the whole is mixed, and each owner receives their remaining seventy percent back in a more compact form. An effort is made to retain some parcel boundaries, as they are useful reference points during the new surveying—so the new parcel does not end up completely in a different spot.
Purchase.
Occasionally, some farmers sell their land to the municipality before consolidation; more often, owners want to capture the increase in value from becoming building land themselves. If the municipality tries to acquire the entire area, this is an indication that they have a [Einheimischenmodell] in mind. Then, the municipality only starts the applicant process once it knows how many plots it can offer itself.