The trick involves the scammer “accidentally” sending you money, followed by a message that says “Oops, so sorry. I didn’t mean to send you that. Can you please send it back to me?”
The BBB says it’s a trick – the ‘accidental’ money comes from a credit card that gets cancelled – invalidating the payment and removing the cash from your account a few days later. But by then, any money you sent them is long gone.