As a renter you pay for the fuel that you use. These costs are not included in the rental price and are separate from the free kilometers (Read more about the free kilometers here). Agreements about the fuel and any settlement are arranged with the car owner.
The starting point here is that you hand in the car with the same fuel level as you have received it. We advise the owner to provide a full tank at the beginning of the rental period so that you can return the car with a full tank. This is the easiest for everyone.
If this does not work then you can settle deviations of extra kilometers driven, by means of cash payment or with a digital payment request at the end of the rental period. You record the fuel level during check-in and check-out.