Put the item behind the desk.
"Sorry, I just need to get an override code, let me go grab my manager, I'm sorry for any inconvenience"
If you know for sure they're trying to con you, they'll probably just leave. If they want to play it out, go get your manager and let them sort it out. As the manager in this situation, if I knew for sure they were pulling a scam, I just told them to get lost. In over a decade I was wrong once, but even then I was right, because the kid who was trying the return stole the items from his parents. They came back in later, explained the situation, and I just gave them the items (sheet sets). Could the parents have been in on it? Maybe, but that was a long con for 100 bucks, they earned it.
I always taught my SD TMs to take all return items and place them behind the desk on any high shrink items. Prevents situations where some shitbag tries this exact scam, then just leaves with it. The scam isn't the return, that's a bonus. The scam is the front of the return adding legitimacy to the theft. They just walk out with it and you let them.