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

Who removed it on both properties on whose order? Is this a new development area?

Everyone has hired the excavation company for digging "their" area, we both have the same excavation company (otherwise it would have been "more complicated"). Yes, it is a new development area.

Attached is also a picture showing the garages, the parking space, and the (still unfilled) workspace.

So, in my opinion, I would also say that everyone has to protect their own, since both of you started at the same time, both excavated and both filled again. But I know that the law sees some things differently. There are views such as that the party causing the disturbance must keep the party being disturbed free from any burden caused by the disturbance. Someone could actually come up with the idea to say that when B's garage was built, there was no soil there. You are filling up your property again, so you have to ensure that his garage remains intact. But I am a layman; a real lawyer is not allowed to provide advice here anyway, so better consult a lawyer before you take a position in court that is untenable.

... when setting up the garage by B there was no earth there.

Of course there was no earth, it was dug up in this area so THAT the garage(s) could be set up there ... If you can and may really argue like that .... then ... hmm ....

After I saw the picture, it became a bit clearer. The excavation pit in the mentioned area was only necessary because of his garage. So he created an excavation pit and you allowed him to slope on your property. Not only because of that is it clear to me that it is his excavation pit, you would have also managed without the last 2 meters. So he has to close his excavation pit and everything else that belongs to it. Either way, not a good start.

Wow, that is quite a difference in height.

How does B secure the garage towards the lawn at the back? Can he just fill it up? The prefabricated garages are both only standing on strip foundations.

Basically, you could also place the L-bricks diagonally towards the corner of the B garage, then he would pay for one more and the cutting work.

But you haven’t answered the question of who commissioned the excavation of the slope yet, have you? It must have been an excavator that removed both, and not two separate ones?