If the amount paid is different than the amount of the sale, you must use the payment method DIV.
Seize in the virtual keyboard the different amounts paid by your customer.
In case of tips, the balance to pay will be negative (amount paid > sale amount). This discrepancy is resumed in each 'Reports/End of day', in the 'Differences and service charge' section (or in 'Reports/Differences and service charge'). Your accountant will manage these discrepancies as tips according to the accounting rules of your country.
To avoid these discrepancies, if you pay back in cash the tips received by your staff; you can complete the amount received on the sale with a negative amount in CASH (to balance the outstanding amount to pay). This amount will be deduced from your cash amount in till.

In some countries, a service charge can be invoiced to the customer. It is often a percentage of the total amount of the sale.
In 'Settings/Users/Access management', enable the right : 'Manage service charge'.
You can set up the service charge rate in 'Settings/General settings/Service charges'.
At the checkout, you can click on the 'Calculate' button to add the service charge automatically in your sale.
You can modify the service charge amount if necessary.