Builder demanding Retrospective Maintenance Charges

Had purchased a flat in September 2016 which is built. I didn’t purchase the flat from the builder but from its previous owner who has purchased it from the builder. The apartment is maintained by the builder as he had not sold out all the flats. The builder was charging maintenance charges at Rs 2.25/ SQFT per month till March 2018. However, in a letter sent in March 2018, the builder stated that the actual rate of maintenance is 2.75/SQFT, hence all the flat owners including have to pay the retrospective difference amount from the past two years (March 2016 to 18) as the builder now wants to form Association and hand over the maintenance to the association. I don’t have any agreement with the builder on maintenance payments, however, in a letter sent during Nov.2017 asking me to clear the dues, the builder in that letter stated that the maintenance charges are at Rs. 2.25/SQFT. I objected to the builder’s demand of retrospective difference amount saying it is unfair to demand after two years and wrote emails to which they didn’t reply. In between the builder has never intimated in what soever way that the charges would increase. Just yesterday, I send another email to the builder asking for a discussion to which they denied and said that I have to pay the difference amount. Moreover, they had never sent any receipt of the payments that I had done in last two year despite repeated emails. Please advice what should I do.