The builder, as well as the society, cannot allocate parking spot in violation of the approved plan. If the BMC has approved 45 parking slots, each for the flat above the carpet area of seventy square meters, then the builder is obligated to allocate them accordingly along with the sale deed.
You are entitled to receive a parking spot for your flat 1503, as per the approval granted by the BMC. The allocation of one parking spot is an integral part of the sale deed if the carpet area of the flat exceeds seventy square meters.
According to Section 2(a-1) of the Maharashtra Ownership Flats Act (Act), the definition of a flat includes a garage (parking slot), which constitutes premises forming part of the building.
As per the approval given by the BMC, each flat above the carpet area of seventy square meters shall have a parking spot. Consequently, the parking spot becomes an integral component of the flat/apartment. Any breach of such approval contravenes the provisions of the Act, and therefore, an aggrieved person, such as yourself, can take legal action against the builder/society. Such an act is illegal and causes irreparable loss and injury to the flat owner, i.e., you.
Charging different fees for parking slot is a separate matter if the vehicles have been parked in common areas. The society/builder can charge varying fees if they are providing additional facilities. However, allocating common areas for exclusive parking is unlawful, as that area cannot be allotted separately.
Generally, the society charges extra fees for parking in common areas based on the rule of necessity (although it is illegal, it is done for the common interest of residents). If they are charging different fees for parking slots, such classification between different parking spots must be just, fair, and reasonable.
You can take legal action against your builder/society (as the case may be) either by filing a complaint with the appropriate authority of the BMC because they are violating the approved plan or by filing a complaint in the district consumer forum due to a deficiency in service on the part of the builder.