This is more or less a newer version of a very old scam, such as the following link: https://www.snopes.com/fraud/sales/cashier.asp
Someone supposedly sends you money. They either give you a check, personal check, or surprisingly enough in your situation claim to want to put it directly into your account themselves.
They only need some of this money, though, apparently - and offer to let you keep the difference after they ship a small portion of it somewhere else.
The scam is that the method they use to deposit the money into your account, usually a cashiers check, can take literal months to turn out to be a fake. Much, much longer than the banks are legally allowed to hold the funds.
So it appears as if you've made off with quite a bit of money; Long after the transactions have settled, the bank realizes it's a fake deposit, and you're left with the difference: It's as if you never had the money, but suddenly decided to send the scammer some cash.
The amount they gave you is reversed, but the amount you sent them is not. So you're left out the cash they claimed they were taking out of 'their' money. Their money never existed in your account; it just took too long for the bank to verify whether it was real or not so they made an assumption.
Scams like this can be quite complex - here's an example of one that may seem fairly legitimate to some until they've already signed up and received a check: Received an unexpected cashiers check for over $2K from another state - is this some scam?