The problem was that it was more like a bank. Person gives you $9k cash, you add that to their account but it doesn't work so you push the button again. Now the computer thinks you should have $18k more cash in the till but there's only $9k there. There's no way for a bookkeeper to know that you didn't take that extra money.
Surely the system must be able to spit out what services and products were sold and how much they would be worth.