Assuming you have the receipt, NONE. Your receipt for a cash sale is your ID....as far as their system is concerned
I have always had them demand an ID for any returns, cash or credit. When I suggested their policy was discriminatory against US citizens they told me the policy is universal and also accepts a Mexico ID.
It depends on area and which stores, the stores that are getting hit hard by theft/crime in their neighborhood have to be tougher. Its not hard for employees to understand when they're seeing it for themselves first hand. Sadly, in some spots theft has been so rampant and they're doing everything they can just to slow it down, not stop it. Dont expect a Laissez-faire attitude in those stores and has nothing to do about you or being discriminate unless that particular employee uses it when he shouldn't.
Generally, you don't have to show ID if you have receipt especially if its being credited back to a card, same with cash with a receipt, they may ask you for name, usually just a phone number and maybe sign something (depends which state/laws) but trying to return something for cash without receipt, then yeah expect to be asked for an ID and asked some questions and info and most likely will receive a gift card for the amount. Which personally I don't see what the big deal is and not discriminate. Be thankful that the company even accepts a return without proof and places cold hard cash in your hands. Thats a goldmine attitude for thieves and asking for ID to reduce multiple episodes from same thief, it doesn't stop it.
Milwaukee Power Tools are high theft items, like a Ford T truck or a Honda.
In a Florida HD where I was working out of, there were dudes hunting for dropped receipts in the parking lot, even asking for people walking out the store for them, once they got them, they would try to grab a matching item on receipt, walk around and try to get cash for it by saying there returning it even if a credit card was used. I'm guessing a lot of legit buyers unknowingly got purchases credit back to them when cashier didn't hand out cash. There they started asking everyone for ID for returns when they realized what was happening. But there's less reason for a store who is having low theft track record to be as strict (ebb & flow).
I think it's a very fair request to show ID if you're asking a company having theft issues ignorer to give you cold hard cash coming out of their hands and into yours.