The import of the exhibition of paintings in Germany

[Seriy]

Intend to import the pictures on exhibition in Germany. Already stocked in the German document that refers to the exhibition of our cultural exchange and inter-state government department of the Russian Federation is carried out in Germany on a commercial basis .
In principle , there is a Protocol of Florence in 1950 on the import of books and pictures and things, exempting these items from customs duties and it is approved by the Bundestag ,

But the German customs officers actually stick with a request to pay a 7%fee. Who knows the thread , on what basis , what law - customs document required to pay a deposit fee ( refundable if it is exported ) and how to avoid the formal ?
Thank you.