CSO (a leading industry reference for news, analysis and research on a broad range of security and risk management topics) has published a useful article on POS manipulation fraud. The article can be found here. The key points are

Retail theft is a huge and costly problem for the industry. According to the “Global Retail Theft Barometer 2013-2014,” released by Checkpoint Systems in October 2014, total shrinkage among retailers in North America was $42 billion for the time period covered by the report. North American companies had the highest shrinkage rate in the world, “as it has the highest concentration of retail stores and significantly lower retail loss prevention spend” than other regions, the study notes. U.S.-based discounters, pharmacies/drug stores and supermarkets/grocery retailers had the highest shrinkage rates, stemming from shoplifting, dishonest employee theft and organized retail crime.

The article recommends the following points to reduce staff theft and fraud at the point of sale

• Leverage the latest technologies: Primarily CCTV, RFID and data analytics. 
• Train your employees well: Staff training should make every employee feel like he or she has a responsibility to prevent shrink from occurring.
• Interact more with staff & customers: The most effective deterrent to a potential shoplifter is good customer service. Making eye contact and acknowledging customers as they enter the store. The last thing a shoplifter wants is interaction with a store employee. Likewise for staff, regular interaction and oversight from management will suppress fraudulent actions
• Create the right corporate culture: Anti-theft technologies and tactics are vital, but they’re not enough. All the tools in the world are not a replacement for the most critical foundation needed to prevent, isolate and resolve retail losses, and that is a corporate culture focused on mitigating loss.