It's a scam. One thing they might do is "accidentally" send you a much higher amount of money they they originally mentioned. You would see the pending transaction in your account and assume that what they are saying is true. They'd ask you to send it back, and seeing that the transaction is in progress they're hoping you do, but in reality it was a bad transaction. They've tricked you into giving money to them, and their faulty transaction never goes through, leaving you out however much money you sent, and scammed.