What does "Fehlbelegungsabgabe" mean?
I didn't find a definition anywhere, I just found a Wikipedia page: Here
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The Wikipedia-page explains it in detail.
The word is combined by three parts Fehl-belegung-abgabe. Fehl is related to Fehler (error). Belegung is occupancy, Abgabe is a fee.
In Germany exist appartments for people with no/low income. The rent is very low.
It can happen, that you live in such an apartment and you find a new job with a higher salary. So you earn too much to have the right to live in the apartment. You can still stay in the apartment, but you have to pay this Fehlbelegungsabgabe. It is the fee to pay, because your occupancy in this subsidized apartment is not correct.