Who has to restore the height of the garages/parking space?

Small update: B has sought legal advice (which is okay, as a layperson one might not be able to assess this properly). The lawyer says that A has to ensure that the (now) existing garage does not suffer any damage from the filling work (on the side). (The garage at the back is ordered to be earth pressure resistant). hmm. I can only explain such a statement from a lawyer (with construction law experience) by the fact that not all details and the background situation are known to them: For example, the situation from 4 months ago, when the entire area was still covered with 3 meters of earth. And the information that the now "free area" next to the garages and the parking space was necessary working space for the garage installation. Furthermore: B has also known for 10 months that the area behind our parking space is to be refilled with earth (email correspondence, sketches) ...

To make it clear again: B is also dumping again?

To make it clear again: is the B also pouring back again?

Yes, at his garage, but only at the back (it is earth-resistant in that area)

The crucial point of the question is whether the excavation has created a new terrain layout in the legal sense. If this is the case, your filling would alter the terrain and entail the obligation to secure it.

I am also just an amateur, but I could imagine that it counts as a joint construction project and thus, without a written shareholder agreement, all costs are simply divided by two or each person bears their own costs concerning their own property, and costs concerning the common boundary are halved. I can hardly imagine that A has to bear the costs of securing the common boundary alone. But at sea and before a court....

What kind of cost are we actually talking about? L-shaped stones and filling, right? You are neighbors and the construction workers are coming anyway with their machines. Or is it about the infamous "principle" again?