...but then you sent an invoice with a EUR price on it. I think this was a big mistake. He can now claim that the price is right there in EUR and that, at the moment of the transfer, he gave you BTC worth that exact amount.
^^this
You said, you made a deal in BTC, then how in hell did you come up with an invoice in EUR?
If i may guess, I'd say it was Trumans idea, If so,
that's where he tricked you out of your Coins.
Anyway, he has a history of screwing people, so you're not alone and I too feel with you.
Thanks for sharing your story.