1) An employer may never take money out of a paycheck without the employee's agreement or persmission.2) If the employee was at fault in accepting the counterfeit bill--that is, he or she was either unreasonably careless (for example, it was a bad fake, which the employee should have spotted; or the employer knowingly accepted a friend's bad bill), then the employee would be liable for the money. If the employee does not voluntarily repay it, the employer could sue, though it would not likely be worthwhile to do so.3) On the other hand, if the employee was not at fault (for example, there was no reason to think it was a forged bill), then the employee should not be liable.3) Of course, if the employee does not have an employment conract protecting his or her job, his or her employer could fire him or her at will, at any time, for any reason--including being annoyed that he or she took a bad bill.