A hacker wants to help me. I send them $400 USD through Western Union (Turkey) and they send me back $8k. Is this a scam?
Most likely, they will send you the $8000 first, as an "act of good faith".
However, they will not send it Western Union nor PayPal gift-mode. They will send it as a purchase, as a bank transfer, or cashier's check.
This will appear in your account, and the bank will release any lock on the funds, and then you will go "Hurrah, they were honest, I shall Western Union the $400!"
And then, after some time, the purchase will be disputed, the transfer reversed/clawback'ed, or the cashier's check will bounce.
And you will go to Western Union and tell them to reverse the money. And they will laugh at you and say "Western Union does not work that way!" Your $400 is gone.
So you are out the $400 and whatever bank charges toll from the failed transfer or deposit.
Also, consider a scenario. Suppose they sent the money from an innocent person's account that they hacked, and the person spotted the transfer and reversed it. The police might have some questions for you. This is where things get rather awkward. By your own belief, you went into a "hacker" site, asked them to do this, and paid them $400 to do this.
You may not have realized what the "this" is, but under the circumstances, you reasonably should have known crime would be involved somehow. That makes you an accessory to the crime.
Which is still a crime even though it's been reversed, by the way. The crime is the trying, it doesn't stop being a crime if you get caught. Obviously.
Yes, This is a very common scam.
https://www.thebalance.com/western-union-scams-315825 explains it from the perspective of scamming a buyer out of goods. This still applies to you because you have $400 of money that the scammer wants as opposed to $400 of merchandise.
You might wonder how it’s possible to lose money if you already received a payment from your buyer. Unfortunately, money does not move through the banking system as fast as you might expect—unless you use a traditional wire transfer.
If you receive a check—even an “official” check or a cashier’s check—the check might bounce several weeks after you deposit it. However, your bank will add the funds to your account balance and allow you to withdraw the money as if the check was good. But you are ultimately responsible for all deposits. Therefore, if the check bounces after you withdraw the cash, you’ll have to replace those funds.
The same holds true for electronic payments. A scammer might send money to your PayPal or Venmo account, and you might think all is well, but those charges can be reversed. The sender could have used a stolen credit card or a hacked account, or they might simply dispute the transaction. Either way, you won’t find out about it until after you’ve sent the scammer good money—which can’t be recovered.