Sold flat to builder itself but he is not making any payment

I sold a flat that I got from the builder to the builder only. Now its 1years 4 months past n the flat is ready for possession but the builder has only paid half of the money and also he didn't gave me the agreement copy.As I stay outside of Guwahati its getting difficult for me.He is neither giving me the rest amount nor is he giving me the agreement copy.I am a bit confused n as far as I remember in the agreement its mentioned thar he will pay the amount within 1year.I need your guidance as to how can I get the agreement and also can I cancel the agreement.I have proof of making thousand of calls n sending messages for payment as well as agreement. N also he doesn't reply my msg or take my call. Plz help me. If anybody is interested I can provide the whole details in email. Request for help