In November, we went to study at a driving school, the course was paid in full ...

(theory and practice, + a lesson for free with an instructor), but since it was not possible to pass the theory because of work, it is possible to pass it now, but the prices at the driving school have been raised, and now they are required to pay the difference, and it is already 6000, on which basis? Gasoline has not increased in price during this time, we initially paid in full, not in installments. Is it legal?