builder sold illegal property

Hi all I paid more than 900000 rupees to builder to buy flat in bhiwandi since dec 2012 till jan 2015 when I came to know that builder has sold us unauthorized building flats with no na clearence and no mmrda approval I stopped making payments and when I tried contacting builder he did not respond and he closed down his office switched off his mobile. I submitted an application with police and now police got hold of him. Builder has given me written assurance that he will return my money in three months ..can forum help me with next course of action .please note I paid most of the amount by cash and builder has not registered my flat