I also think it's intersting that a WalMart employee would be predisposed, or automatically assume, that their customers are dishonest, or are likely to be pulling a 'fast one'.
Then you have no idea how many scams are run on big retail outlets daily, the craftiness of the criminal mind would astound you.
Lets do this again and see if we can get it clearer , If he had come back in out the parking lot on the same trip he bought the ammo with all 5 boxes in tow that would have made an immediatly believable scenario.
If you think the following scenario is far out then you have no concept of retail loss prevention in a large outlet and this has nothing to do with the actual product purchased , could be ammo or hairspray:
Joe Shyster has a reciept for 5 boxes of ammo he shot up last week and he needs some more but he's tired of paying $19.97 for it so he sees the .40 for $14.97 and gets him a box, walks outside to de truck and leaves the $14.97 reciept in the truck and grabs the week old $19.97 reciept and walk back into the store and right up to customer service and tells them he was given a wrong box, and he wants the .45 that he paid for. Ok so they do it for him , he just got a box of 45 for $14.97.
Based on what some Police type folks have learned from busted scammers , I'd bet this scam is run at least 5 times a day at your local WalMart , sometimes by "crews" who have several ID's for each crew member and know when shifts change and such. No it's probably not run on ammo , but add the liability policy factor of ammo returns and it becomes much easier for the clerk to throw up a red flag , and you and I are safer for it.
As to the liability , think about it , Tamperers will take their time to learn what their doing as evidenced by the Tylenol tamperings a few years back.
Do you actually think there are no anti-gunners who are rabid enough to have thought of this , to discredit us by blowing up our own guns in our faces ??? Nevermind the cheapskate schmoe who just wants to fill the box with cheap reloads , they are a secondary problem. The world is full of bad people , thats probably why you own a gun in the first place. Why is scamming so hard to believe ?