Cashiers don't normally have the ability to do cashback from their own register, the customer has to initiate it. But also, there are places like our RiteAid downtown, where if you tell the cashier you want cashback, you enter your pin# on the pad but then the cashier has to enter the $ amount into the register. The customer doesn't get to enter the $ amount. So there is a potential for problems there I suppose.
RiteAid is the only place I have seen that, not sure why/how it works that way. But the cashiers there can't initiate cashback transactions on their own kwim, the customer still has to select cashback on the pad first...