I don't think it's as false as Snopes says...

I will say I personally eyewitnessed this "cash back request" when it was not asked for, and an inquiry made about it, at which point the cashier simply told the customer they had problems with that happening and simply handed the cash to the customer.

Was it a technical glitch, or now that I read that post, something more sinister.... only caught when the cashier was called on it by an observant customer?

I'm not relying on Snopes as "The Internet Fact Checker". They have been wrong on a few things before, some of which are politically motivated.