The OP's initial error was renting the 4WD vehicle in the first place. When he saw its condition, he should have walked out. I know it is hard to do when on vacation, but sometimes you have to.
As a long time driver of 4WD vehicles, I can say that most people have little idea how to drive them. In any case sand is a bad situation to get out of even with experience.
I'm afraid the OP will have to pay and will be lucky if it is not more.