There's always thievery, which has absolutely never failed for anyone, ever.
You could try your luck at gambling. You'll get that money in no time!
Alternatively, if you're into doing things outside the box, you can kill the person who you owe money. With that person gone, not only do you not have to pay them, but you couldn't even if you wanted to! Note, this also works for banks, credit card companies, even governments, you just have to kill many more people.
Seriously, though, you probably have close to $300 worth of stuff you can afford to get rid of. Looking at your profile, I can see that you have an Xbox live, PSN, and Steam account, so you probably have an Xbox, a PS3, and a computer, along with games. You can afford to get rid of all of these, except maybe not the computer (maybe being the key word here).