This question (How can I prevent my employees from gaming a reward points system?) was closed as off-topic because in its original version, it seemed to ask for an app design.
The OP clarified in the comments and edits that their actual issue was about preventing the cashier from gaming the system, which seems to be on-topic.
I have edited the question to remove the details of the app features and rules, in order to focus on the main on-topic issue.
Please consider casting reopen votes if you agree.