Either
- Don't take a car that is not full
- have them record the actual amount when you take it out and bring it back with that amount in it (I know for some here it's like advanced calculus in the dark with only soft sand and a stick to do the numbers in to figure out how to do that but for those who can it's pretty easy)
- Ask they fill it up before you leave the lot
Not sure why you feel you are being penalized if you bring it back and they have to fill it up at $9 a gallon, you agreed to that price when you took out the car and you made the choice to not return it full.
Expecting them to reimburse you at $9 when you paid a third that is just plain silly. Let us know how it works out for you.