Well it's not something so strange, and I see no difference with what's happening every day.
People return stuff purchased for cash all the time for bunch of reasons, paypal cashbacks, people don't like bike, people changed their minds, card declined, tainted stolen cash bills, fake cash bills...
Maybe guy who pays don't even know he is using stolen marked dollars, so he needs to accept return and pay with real dollars.
Not all of that is a
cash return. In particular, anything to do with cards or PayPal is dealing exclusively with banks and doesn't touch cash at all.