Hmmm...I can actually see this going somewhere. Someone offered you $500 for this? What's your email address? I'd like to possibly talk to you in private and see if we can work something out.
If this fight gets going, it's a 95% bet that yours will be the best term used. FloydvsManny.com is already up and running but people are already more comfortably saying manny vs floyd... and once a phrase takes off a certain way, that's the phrase people search.
If you have no development plans, maybe talk to your Phillipines buddy about some kind of a shared development/shared profit -- with you keeping control of the domain name and revenue account, of course. Or try work the same deal with someone at namepros. Do it carefully and with someone trustworthy.
If you want the quick cash just sell for that price and make peace with the fact that it looks like this fight will happen and if it does, the owner of the name will make a good bit of cash for awhile. It's all about pros and cons, right?
FYI if I had this name, I'd do some minor development with a minisite, some boxing news feeds, some updates. Maybe make it a simple blog with some adsense...
Because when these events happen, you can make quite a ton of cash on ads, if your site is indexed well and has some good content.
I'd start now.
$500 isn't a bad price, not like someone's ripping you off, they're taking a chance too... but IMO I'd rather take a chance on making a lot more than $500, and risk losing the 'sure sale'.
Just my 2 cents, from my situation.
Only you can decide what you need in your situation.