I just watched ABC Primetime, which used hidden cameras to test bystanders. It found that when a man is abusing a women in public, a female bystander is twice as likely to intervene as a man. But when a woman is abusing a man, the female bystander usually says that the man deserves it!
Then it gave people extra change in a fast food restaurant. Many returned the extra bill immediately, and others seemed not to notice. For the latter, the TV crew confronted each one and told him that he got an extra bill as part of a TV stunt, and tried to shame him into returning it.
If I got extra change by mistake, I would feel some obligation to return it. But if a TV crew told me that it was all a stunt, then it was not a mistake. Those people were being paid to participate in a commercial experiment, and they have no obligation at all to return it.
The only reason to return the money would be that some nasty TV producer might try to make you look bad on TV if you don't. Okay, that is a good reason. But there is no legal or moral obligation to return money that was deliberately paid to you.