If you notice too late to just get the vehicle changed or don't want to wait for a vehicle change, go into a petrol station, fill the vehicle up with petrol/diesel and get a receipt

When returning the car , hand the receipt as proof that the vehicle was not full and get the amount removed from rental cost.

Alternatively if, as most cars I rent have these days, there is a "fuel used" display, reset that display and when refuelling , just refuel the number of litres/gallons recorded as having been used plus enough for the drive to rental location

Quite simple