Developer | Special Request | Cost Assumption

Is it even a developer or a general contractor, in other words, does the land belong to you? As the OP describes it, the contract award was made verbally, which is not uncommon, rather the opposite.

When building with the developer, we as buyers are normally not authorized to give instructions to the subcontractor.

Furthermore, the developer does not have any payment claims against the client.

You do not build with a developer; you buy from a developer (possibly something that still needs to be built). From the buyer's perspective, there is no subcontractor involved; there would be one in the case of a construction contract with a general contractor. Likewise, there is no client in a developer scenario (or the developer would be the client themselves).

The terms will be added later.

In principle, my answer applies to constructs within which a person has a [HausbauV] with a [GU] (at least that is how I understood the OP).

Sorry, then I should clarify that again.

There is a property developer contract. Not a general contractor.
We were allowed to visit the house and a craftsman commissioned by the property developer asked us about the corner rail and plaster. (He knew that we are the buyers)

Hope everything is clear now?!

Ok, in this case: Craftsman asks if anything else is needed. Buyer agrees. Craftsman does it and shows it to the BT that it has already been done. BT issues the invoice.
Question from me: why do you have a problem with it now? Just on principle? Did you want to do it yourself? Who else would have done it?
How high is the invoice for exactly which items?

why do you have a problem with it now?

On the one hand, because we had asked the developer in advance for an offer for these positions, but received no response.
On the other hand, because as buyers we thought that special requests are regulated in advance by offer according to the purchase contract. (We assumed that we do not have any authority to issue instructions, as it is currently neither our land nor our house)

Did you want to do it yourself? Who else would have done it?

Either our stove fitter, ourselves, completely omitted the corner rails depending on the price, or awarded the contract externally after the house was completed. We would have liked to make the decision ourselves.