Be careful of doing this deposit/withdrawl method even if the site allows it. Most (if not all sites) reserved the right to cancel your bonus, etc if you are "abusing" their bonus offers. When they offer to match $x, the intent is that you will deposit and play with $x dollars. Depositing $x, getting the pending bonus of $x on the books then withdrawing $y is easily considered "abuse" as you'd obv be trying to get something for nothing. I understand that technically you'd be playing by their rules (unless they specifically state that a withdrawl with pending bonus negates the offer), but it's obviously just to get over on them. Most sites I've seen also state that if a new deposit bonus is offered and you withdraw money before depositing for the new offer, then you void your eligibility for the new offer.
Best advice: If you only want to have $100 on the site, then deposit $100 and get the $100 bonus to work at. Besides, if you're following correct BRM methods, you'll have an extremely difficult time clearing a $500 bonus with only $100 on the site due to the limits you'd be playing by BRM guidelines.