Builder not wiling to return balance maintenance charges to Co-op Soc

Dear Team, We paid advance mainteance to the builder before society formation to the tune of 1260/- per month for one year. After society formation, builder (according to society members) increased maintenance charges to 2050/- per month and has deducted relevant amount from balance maintenance. We as an individual did not received any communication to this effect from builder. This is undue hike and builder clearly wants to retain as much as possible fund with him. Builder is reputed and this is the first transaction issue from his side. Please guide on what legal course do we have at ahnd??