Catch with retaining wall - height specification

How do I determine the original ground level when the neighbor to the south of my property has removed material on the first meter towards the north, so that no debris falls onto his property?

Did you allow that?
If there is a development plan, an original ground survey should have taken place. If a proper site plan was prepared for the building application, it should also include the original heights. If you cannot prove that the terrain was different before the construction measures, the terrain found at the time of your building application will be assumed. In case of doubt, take meaningful photos of the construction site, preferably with ranging rods at the boundary points so that the height differences are visible, and discuss the whole matter with the building authority.

If you want to support higher than 2m, either the neighbor must agree or you must comply with the setback distances. Usually, strong terrain modifications are not welcomed, so the legal means for refusal are mostly exhausted.