The cash dispenser part is probably just a black box with a simple interface that accepts a message to "dispense some amount of cash" and that can respond "request failed" or "request succeeded". It may even have no concept of inventory and just knows that it tried to dispense X bills and it only got X-1 of them.

Yes, it could have a query function added, and yes the GUI could dynamically respond to that, but it's a bit of an edge case since the assumption is that the ATM is well stocked if it's got mixed bills.