It is a scam. Guaranteed.
If they really sent the $5k keep it. If you send it back, the original $5k they supposedly sent you will somehow disappear and you will be out the $5k you sent them.
I would keep it. If its a scam, you likely won't have it long. If its not a scam, they can wait 10 days for the money transfers to clear to confirm it wasn't a scam.
This scam stuff happens a lot.