But when it came to the invoice, you put a price in EUR on it. If you didn't mention EUR or USD while negotiating, then how can he know it was not pegged to the price in EUR? It was never mentioned after all.
Don't get me wrong, I feel with you. From your point of view you made a deal in BTC. Just that, nothing else and thats fine. However, it seems like it wasn't that obvious to him. Until here, I see no problem. If things happened the way you described up to this point, then its clearly his fault for not adding a EUR pegging into the contract before agreeing.
...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.
In my opinion you should have created an invoice in BTC. That way you'd be safe from him claiming it was a EUR pegged sale.
What is the applicable jurisdiction for this?