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

And, a small addition, I informed the dear neighbor in December 2022 (via Messenger), that 2-meter L-stones were necessary there, so that wasn’t a surprise, but maybe he never responded to my info with corresponding ulterior motives...

That totally contradicts his lawyer’s statement that they had "mutually" agreed on the new height level (i.e. "below") as the new level...

And somewhere I don’t know, as much as I am interested in maintaining a good relationship with the neighbor (he is not technically savvy, and I have already helped him a lot and given him tips during the construction phase... for example, he could have taken a Telekom cable connection for €1000 instead of free fiber optics, etc...),
whether one should just give in to such people... (if you really have 99% right (if I’m wrong, no problem then I’ll pay for the L-stones!))

The lawyer writes down what the neighbor tells him. Since it has now come to this point and the lawyer is sending you letters, I would also not respond to the letters yourself and by no means present any counter-evidence on your own, but leave that to your lawyer. Unfortunately, it looks like one can no longer have a normal conversation with him. Otherwise, I would have suggested a 50/50 split for the sake of peace. Alternatively, you can still do that, but I would also only do it through a lawyer and without acknowledging any legal obligation.

I remind you again of my comment.

The crux of the question is whether digging out has legally created a new terrain profile. If that is the case, your filling up would change the terrain and entail the obligation to secure it.

Something similar must have been told to him by his lawyer; accordingly, that is the approach being taken now. The new crux is to get out of the situation again without becoming eternal enemies.

What did the neighbor reply to that back then?

He did not respond to this info, had written two pieces of info/things back then ... he responded to the other one ...

I can only repeat, it was actually clear to everyone that soil would be placed there again, therefore both garages are earth pressure resistant (at the back), and our L-blocks at the parking space, and always the 2m L-blocks on his garage side wall in the sketches (which I also sent to him) ....

So, the way you present it here, he wanted to deceive you from the very beginning. I would be upset too. Even though our understanding of the law and sense of justice are aligned here, as I said, you should better seek advice, because especially in construction law some things are not at all handled the way you would with common sense. See his lawyer's letter, although I also think that he probably presented the facts differently, or left out the beginning.