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

Costs, estimated by the excavator at 4000 euros (only the 2m length area of Garage B).

It's not about the principle, or what do you think?
Why should someone participate in something if it is "clearly" (if that is the case!) the other person's responsibility?

The crux of the question is whether a new terrain profile has been created in the legal sense by the excavation. If that is the case, your backfilling would change the terrain and entail the obligation to secure it.

Why should that have happened? If the new profile is not above the old one, definitely not your problem. Talk to the neighbor, tell him that you will backfill up to the old terrain profile and give him a reasonable amount of time to react. If nothing happens, go to your lawyer, set a deadline of 2 weeks in writing, and charge him for all additional costs caused by the delay; otherwise, some people don’t understand.

I did not read very carefully who is A and who is B. But if the excavation was carried out jointly and both knew from the beginning that after the construction of the garage and parking space both sides were to be refilled to the original level, then it raises the question for me why B, with this knowledge, does not also have a garage built on the side that is earth pressure secure and now passes the responsibility to A. Maybe this is legally more complicated.

If the process and the excavation had been independent of each other, it would be clear to me (warning, just a layperson’s opinion!): If B had excavated first and built the garage, he would have had to secure his garage laterally against earth pressure because A would then have been higher. If A had excavated first, he would have had to secure property B for the duration of the work and then could have refilled normally, and if B later built a garage below ground level, he would then have had to build it earth pressure secure on the side.

I see the excavation as a means to an end for the installation of the garages or parking spaces. In this respect, it would not even be considered a change. And without sympathizing with A, I assume that B did not explain this to his lawyer.

Honestly? - 4k opportunity costs are peanuts in the context of the total construction costs if they serve the course of determining the neighboring relationship. Even a dimwit is more pleasant if he is peaceful. Be wise like King Solomon and offer to share these 4k expenses fraternally. Present the deadline for his consideration time as "practically for the order placement if he accepts the offer within two weeks." You always need a cup of flour, a jump-start, or a snow shovel. Leave it to him how wise he is and let him decide freely.

Got new "info" from the neighbor and his lawyer ...

> ... after soil was consensually removed before ...
Sounds good, however, "only" a necessary excavation for setting up the garages and space for the parking spot was created.

What is especially unclear to me is where we "bindingly and consensually agreed on the current ground level."
It was excavated accordingly, but no "agreement" between us regarding a "new height" or anything like that was discussed or made ...