Just don't use the "change buddy?" guy who is usually camped out in front of the cambio at the roundabout. He'll try to burn you if he can. One time I had him get right up in my face and yell "hey f**k you mon" when I refused to use him (cambio was closed). I had already been burned by him once, even though it wasn't for much $, wasn't going to try my luck again.