The most effective preventative control to reduce losses due to discrepancies between inventory and sales, suspected to arise from the abuse of cash register refunds and voids, would be to require a manager to use a reserved register code to approve voids or refunds. This control introduces a level of oversight and accountability, ensuring that refunds and voids are legitimately and appropriately authorized, thereby reducing the likelihood of fraudulent activities.References: Institute of Internal Auditors (IIA) - International Professional Practices Framework (IPPF)