Loophole for theft - moving items

[GenesisB]

We have picked up that cashiers are reselling a single item multiple times by moving an item from one ticket to another before closing the first ticket. They print the receipt for the first ticket and present this to the client for payment. They then select popular items (such as pizzas / coffees etc. which are harder to stocktake) on the ticket and move them off the ticket to another ticket that ordered the same items. This then reduces the amount of the first ticket on the system and they can pocket the difference (cash) or allocate the amount as a tip (credit card). The movement doesn't show up on any reports so it is difficult to even track what items were moved between different open tickets. 

Is there anyway to stop or track this?