I received a $5,000 sign on bonus. With taxes and everything taken out I received $2872.50. I haven't cashed the check and will be returning it within a month or so of my hire date, which is still in the same fiscal calendar year. My question is this do I have to pay back the $5,000 or can I just give back the check for $2,872.50? Since they can easily adjust my W-2 Gross for year end and they haven't actually paid the taxes on it yet since they are paid quarterly or annually?
Any thoughts on this? I would greatly appreciate the help.

If the bonus is returned, the employer can adjust the amount of taxes withheld in subsequent quarterly tax filings (Form 941) and make the adjustment on your 2007 W-2.

But track it carefully. Mistakes do happen.