It's one thing that they credited your account, and it's quite another if you can dispose of that money, that is, withdraw it without problems and whenever you want. The balance on your account doesn't have to mean anything to them, except just some number and they can probably manually enter the amount they want.

Fair scam  . In many small transactions, they will collect a sufficient amount for them, and at the same time, none of the scammed people will be very persistent in asking for the money back. It's only $4 though