Cost coverage for fencing that is not desired

I don't quite understand that right now, maybe you have a clearer recording so that it’s easier to imagine. Did he now pile up against your L-bricks and then set his own curbs against them? If so, he could have used your L-bricks and screwed his fence on after an agreement, but he probably didn't want this intermingling, which is also okay, since then everyone has their own stuff. I think he is more useful than harmful if everyone has their own stuff here. I would visually separate it with plants or similar privacy screen where necessary, and if you also use this concrete base as a foundation for your purposes, that is completely fine, since it stands on your property.

Who knows... but honestly, by now I don't really care anymore. However, the action does leave a bit of a bad taste..

Talk to him about it; if you don't clearly set boundaries, he will exploit it more and more.

You might find that funny

No, I don't find it funny, it was just a warning of what would come, and unfortunately, I was right.

As far as I can see, it fits after all?!

Strictly speaking, the concrete wedge would have to remain entirely on the neighbor’s side. However, I would turn a blind eye as long as he doesn’t ask for a cost contribution. If, however, he dares to do that, I would casually say that he can have the concrete wedge removed and then get half of a chain-link fence paid for. That would settle the matter.