Flat purchase agreement

Dear All,

I have booked a flat in suburbs of kolkata by making 15% down payment of the total property cost. An agreement was made by the builder on a 10 Rupees stamp paper on 12 th May 2012 and it was jointly signed by the builder and me on the same day.However some of my friends are telling me that any agrement made on 10 Rupees stamp paper is valid for six months only. AS THE AGREEMENT WAS MADE ON 10 Rupees stamp paper is void today. Kindly suggest me how to proceed in this case.