If someone is at a bakery and wants to buy a (overpriced) sandwich, I guess it is normal to expect a correct change in return. If some cents are missing and the customer kindly gives a hint, that the change was not correct, the employee definitely shouldn't do a conversation in the following way:
I gave you the correct change!
- You gave me back a 10 Swiss Franc bill, twice a 2 Swiss Franc coin but there's still missing 20 cents.
I already gave you that!
- Ma'am, I have your change in my hands.
I don't believe you!
Maybe I should use this strategy at work as well. If someone calls with computer problems and I get a YES! on my 'Reboot-Question', I just shout 'I don't believe you!'. Although I doubt, that this strategy is very well thought of and will work...