I don't think they can fine you for the mistake. However, there ARE consequences. If you have not had till trouble before, they shouldn't be legally able to fire you either. The norm for punishment is to first have a coaching conversation and verbal warning. On the second time that warning becomes a written one. On the third you are generally put on some form of counselling and probation. Upon the next problem or similar they would then have the grounds to dismiss you.

In the future, please stay alert.

It helps to count back change. I believe everyone handling cash should do so and it bothers me that people don't know how to.

Say the transaction was for $2.65 and your customer gives you a ten.... You would hand back 35cents and say to the customer, "that's three" hand a two "that's five" and "five is ten dollars". If you practise good cash handling as this such a mistake would never happen. Avoid then simply handing over what the till says to - makes sure to count it back.