I saw this thread: What should I do if I lost money in a scam? and one of the comments remarked it may be a double-con, where the real scammer has talked one victim into opening a bank account to receive money from other victims, and then use Western Union to transfer money back to the scammer.
The use of Western Union for advance-fee fraud, and fraud in general, is well-established - and people maintain that once Western Union has sent the money it's gone for good.
I've never been happy with that answer.
In the US, the cheque and ACH system is such that money can be recalled seemingly at any point. The system does not have a "positive acknowledgement" of money transfer, only failure messaging (which is why banks can't tell you for certain if a cheque has cleared or not - only if it bounced, but 95% of cheques bounce within 3-5 business days, so banks often assume money is "safe"/cleared by then). Cheques and ACH transfers can also bounce in a cascading manner even if they earlier passed okay, for example, if you (Bob) send a cheque to Charlie using money you deposited from a cheque from Alice that subsequently bounced - then the cheque to Charlie would also bounce.
Once you send money to Western Union as cash, it's still there in their cash register after it's "sent" - Western Union doesn't have a teleportation machine, they just voluntarily "release" money they already have at the recipient's end at whatever the current exchange rate is. I understand it's the same when giving Western Union money electronically - they have their own messaging system that's independent of SWIFT and other interbank networks, so if you send USD to Nigeria, that USD is still sitting in Western Union's bank account.
Surely if money could be recalled from Western Union in the event of fraud, then Western Union would certainly take more proactive measures against fraudulent use of their service - as it is, I understand they're actually profiting from fraudulent use as they collect their transaction fees regardless.
For comparison, I understand why cryptocurrency amounts cannot be recovered because blockchain systems are based on an immutable, irrevocable distributed ledger - which is what puts trust, and hence value, in the system, - but ACH and Western Union are not, so there is no technical reason money cannot be extracted from Western Union.