Soldato
Small purchase with cashback, made up story about no pin. That'd be enough IMHO for getting the manager/superviser over, they can then decide on the name thing so you're not accused of being "racist". Just tell the guy "Sorry mate, it's asking for the pin, but my supervisor will be able to over-ride it so you can sign instead".
Surely you've been trained and told the store policy on this?
Also just remembered that you can't get cashback on an Amex card! I don't have one and I know that, so surely the customer, if it was his card, would have known that he can't get cashback on it?