Builder Charges Maintenance from O.C. date

Dear Sir/Madam, I bought a flat in the year 2013 by a builder in a new construction property in Mumbai. The building received Occupation Certificate in April,2013. I gave first payment token in November 2013 and registered agreement of flat in December 2013. I got possession letter in December 2013 but builder charged me Maintenance for 2 years from O.C. that is from April 2013 to March 2015 and mentioned the same in agreement. During signature of agreement when i raised with builder for maintenance he said its uniform for all flats and didn't allow to change the agreement and asked to pay maintenance for full 2 years from April 2013 to March 2015. Also builder charged for VAT 1 % of agreement saying its compulsory. After learning recently i questioned builder for VAT refund as i bought the flat after O.C. after rounds of talk he agreed; simultaneously i questioned for refund of maintenance but builder replied that he has mentioned in agreement maintenance to be charged from O.C. hence he will not refund though by Possession letter is of December,2013 and first payment / token of November,2013. Request guidance if i am correct to ask refund of maintenance and VAT and if builder refuses then request guidance how to handle the matter and which body to approach.