The question needs clarification. Are you asking:
1 - How can you make sure the cashier closes the drawer after payment?
2 - How to prevent the drawer from being open after the sale?
Answer #1 - You need a smart cash drawer which the software supports. It knows if the drawer is open or not. And the software will not start another sale until it's closed.
Answer #2 - The drawer open commands can be removed from the sales screen. Also, the smart drawer would prevent this.
Also, another measure of security you can add is a video camera that records all the activity on the register.
