In "at will" employment, a company can set the conditions of the workplace much as it sees fit. This includes penalizing an employee for having an unbalanced cash drawer. This is true unless this violate the terms of a union agreemnt or employment contract, or if this action constitutes some form of legally actionable discrimination. That having been said, the penalty can't be taking money from the worker's paycheck, at least not without their express written permission. If the employer wants any shortages repaid, they will either have to work out a repayment arrangement with the employee or sue them in small claims court.