Stoppage Of Maintenance Services Illegally By RWA

I have purchased a flat in Rajarhat, Kolkata, and got possession on 04/04/2019. The builder took advance maintenance of 12 months plus GST from me from the date of possession. But the builder handed over maintenance to the RWA after 2 months on 15/06/2019. As per the deed, the builder was supposed to refund the remaining maintenance amount to the RWA. The builder also responded in an email query that the remaining advance maintenance charges will be refunded to the RWA. As builder maintenance charges were more than the RWA maintenance charges & as GST is not applicable if maintenance is done by RWA, the balance maintenance charges if received by the RWA can be adjusted for more than 12 months till July 2020. But as per RWA, the first year maintenance charge is fixed irrespective of maintenance done by the builder or RWA. How is this acceptable? Is this a legally tenable argument. This is nowhere written in the Deed or Sale agreement. What should be my Next Step?