There is no duty if the item is shipped back for repairs (unless the distributor is dumb enough, to declare that the product will be upgraded, which will increase its value ...).
But if the product comes back 'repaired', with the same serial #s, there are no extra costs involved. Only some extra paperwork. Been there, done that (shipped form the EU to the US, and then back).
I don't blame you, but you are asking the dealer to sign a document that makes him legally complicit in tax evasion. I can imagine some may be reluctant to do this. Unless I lived in Italy or Greece I would be as a dealer.