There are many frauds widely used by people to steal at the point of sale. Here is another one that you need to address. Its formal name is the banana trick.
Your point of sale software uses barcodes to recognise stock. However, this does not alleviate the responsibility on your operator from checking that the bar code matches the stock item. People have been caught switching barcodes from less-expensive items to dearer items hoping to pay the lower price for the expensive item.
The only way of stopping this is for your employee to pay attention at the cash register.
Note a variation of this fraud is where your employee purposely scans a cheaper item to allow their friend to get a good discount. The best solution here are cameras.