What can the seller stipulate in the purchase contract?

Right of repurchase can be agreed upon between private individuals even more strictly than by public authorities. If the seller has something like that in the contract, it looks bad. If it is only a verbal agreement, you have to reconcile it with your conscience.

If it is only a verbal agreement, you have to settle it with your conscience.

I assume the presumption of innocence and derive two working hypotheses from it: 1. the other party's interest is legitimate; 2. meeting it does not have to hurt.

It is usually called the development plan. Private sellers do not get far with such requirements, but the municipality is allowed to do so, and that is also the intention,