Getting fake bills at an ATM is unlikely but possible (the people that count and replace them are well trained, but a good counterfeit could get through). If you do get them you can complain. You will have to fill out some paperwork (forms for the Federal Police division of the Central Bank), but you'll get your peso bills replaced. If you get under changed you can also complain and you should get your difference. In this case, the paperwork is only with the bank as the Federal Police is not involved. In both cases, be sure to keep the receipt the ATM gives you. Counterfeit dollar bills are confiscated and not replaced.