If a tip is given, then we assume for the current function that you have a separate tip jar.
That means that when someone pays in cash, you can immediately keep the tip separate and put it in the tip jar. This page explains how you can register the tip if it's received in a different way, for example with a PIN payment.
In the example below you can see a service of 39.95 euros. I have made it 45 euros (the amount including tip) and clicked on "PIN". Then I enter the overpaid amount again as a CASH expense.
For example, the 5.05 tip of this transaction is not added to the turnover (and therefore not charged extra for VAT), but it is registered.
In the cash register layout (drawer) it will look like this:
This is because 5.05 has been added to the PIN and 5.05 is then withdrawn from the CASH to put in the tip jar. The totals stay the same in this way.