What scenarios are important to test in the cashier app?

Question

Software testing

Good time of the day!
There is a training task: an application that, after entering the amount of money given by the buyer, calculates the optimal change option (the amount of each banknote) from the cash register (it is assumed that the data on the number of banknotes in the cash register is error-free)
Please suggest which scenarios are more priority to test and explain why? Thank you very much in advance :)

Answer 1

We take technical specifications We
divide them into cases
We write tests

Answer 2

The most important check is that the amount of change cannot be greater than or equal to the one given by the buyer. I hope there is no need to explain the priority here?
Think for yourself which delivery options are more profitable for the store, which options may not be acceptable.