FPL Hyundai — worst dealers!

We bought an i20 elite car from fpl hyundai tambaram and paid an extra amount by mistake. But they are now not returning that extra amount and they keep on cheating us for nearly 2 months. Is it that difficult for them to return our amount to us? Also, they are not giving us car cover which they promised to give as compliment. Also, the seat cover they have put for our new car is of very cheap quality and they cheated us in this too. We're totally disappointed with fpl hyundai. Worst dealers ever met!