cheated while travelling

I asked the pump employee to fill my tank for 500 rs but he deliberately poured the petrol for 100 rs and said "oh i heard you wrong" then he started filling my car's tank and filled it up for 400 marking and said yes i am done .I asked him you have filled for 400 rs only then he starts to fight no i started the meter again and i have filled for 100 rs earlier and now for 400rs and i understood i had been cheated.Beware of these thugs at petrol pump and see zero everytime you go fill up the tank.