Cheat by builder

Dear Sir, We a group of 6 people had booked a property in Mumbai , panvel location .We had booked flats in the project in the year 2015.The builder had committed that he will be completing the project by December ,2018.As a token amount for booking each one of us had paid amount in the range of 4 lakhs to 6.5 lakhs.The builder didn't start any work and was awaiting for project approvals which he never got.In Jan 2018 after waiting for almost 2 years we had lost all faith on the builder.So we applied for cancellation.But builder asked us to wait for seven months till July for returning our payment.We all waited till July and in July he gave us pdc checques of 2 lakhs for the month of August and September and for the remaining amount he told he will call for a meeting in end September.The pdc which was given to us for the month of August was dated between 22nd to 28th Aug between our friends.So when my friend who was to deposit the cheque dated 22nd Aug he got a call from builder asking more 15. To 20 days time and gave excuses that he doesn't have money now.One of my friend whose cheque was dated for 24th dropped the checques.and also got a dishonour letter.We all want to drop.our cheques and go in a legal way.Pls suggest the way forward.