The error could be that it was correctly awarded by the RNG, but not deducted from the pool. In this case, manually removing the win from the pool would be the answer, not taking it from the player.
How can Coral be 100% certain the error was an incorrect award over an incorrect resolution of pool balances during the clear up. If this is not a network pool, it seems more likely that the award was correct, but the accounting failed to remove the payout from the pool, leaving the money in 2 places at once.
Oddly enough, this kind of communication failure is common during deposits, where the money can leave the bank/card/ewallet, etc, but not automatically credit to the casino. We don't hear the casino claiming the transaction was never legitimately triggered here, we are told it WAS correctly triggered and authorised, but a failure in the casino back end caused the money not to show in the players' account.
It could be the same here, the RNG awarded the progressive, it showed to the player as won, but THEN something went wrong and the payment was never resolved by the back end, so it appears not to have been won if looking at the pool value.
Well said vinylweatherman -- I hope the OP will fight the casino's decision. Please keep us posted!