If the money was counterfeit, then either (a) the cashier would have discovered it immediately and not turned on the pump, or (b) the counterfeit would not have been discovered until the cashier reconciled the cash drawer at the end of the shift (or, even more likely, until the bank processed the store’s next deposit), and the store never would have known which customer passed the counterfeit bill. People who pass counterfeit bills are, almost without fail, either caught in the act or not caught at all, so this woman’s mask would have had no effect.
It's doubtful that a counterfeiter would call 911 for assistance with payment (you never know), but it should be the stations right to refuse to do business with anyone wearing a mask or any other garb that hides their identity.
If the policy is in place, then it should be applied to all customers.