The bank don't have a valid authorisation for the cashback,
so I'd expect them to refund you the money. Then they have
to reconcile the difference with the merchant.
At the moment, you are sorting out the bank's problem for them.
Bank procedures for dealing with credit card fraud are probably
cleaner and quicker, because of the consumer credit act, and
because credit card fraud is more common than LASER fraud, but
I still think you have a case against the bank even if the bank tell you
to deal with the merchant.