...under the intention that you sold me the fake(cz) as real... At minimum, I would demand a real diamond in it's place. At most, a full refund for the diamond.
From a seller standpoint, assuming he was in the wrong, intentional or not... The "return/swap" transaction would have to be done within a reasonable amount of time. If the return transaction is prolonged, the seller can might insist the customer swapped the "real" diamond for a fake. If there's no prior history of "theft," anyone in their right might would side with the seller.
I've learned never promise the world UNLESS you can deliver. Only deliver what you can. Anything over is whip cream.