It's not my money. Most of his money comes from libertarians around the nation. They like having libertarian ideas propounded at that level, even though they know he has no shot to win.
As I said, his not giving the money back is a mistake. It detracts from the message and distracts people into tangents like this. Were he a racist there would be far more than what we have. Of course, the action assures he'll never win, but then he wasn't going to anyway.
Seriously, way too much energy is being spent on attempts to destroy a candidate that won't be getting close to any office in the Executive branch even without such attacks. The amount of energy expended on this guy is amazing.
However, if all his money came from these guys the trail would be obvious. That only $500 from this one guy has so far been questioned is quite amazing actually.
Now, excuses like "giving the money back would be an endorsement of all the other people who give him money" is ridiculous. I understand why people want him to look as good as possible, because such distractions detract from the message. I won't make such excuses, when you find out a declared racist is giving you money you give it back and politely inform him that such support is not solicited or wanted.