Answer:
The fraudster is basically using forged or fake checks that if they are lucky enough will be paid by the bank. So if you deposit the check (and the fraudster is lucky) in a couple of days you will have $3,000. Then you transfer $2,500 to the fraudster's account and keep $500 as your pay. But what happens when the account owner tells the bank and the police that the check was forged, or stolen, or fake? Then you will be in trouble and will probably end up returning the entire $3,000.
The easiest way to check if this is a scam or not is to report the check to the bank and make them verify if the check is good or not.