service tax on ready possession flat

Hi I have purchased the flat at line agreement done on 2nd Feb 2012 possession given in same month and completion letter date is 7th march.the flat was ready possession. Now builder is asking us for service tax and giving threat that if you will not pay the same I will not form the society and sale deed . Kindly suggest what to do? Even builder has taken the cash from me and not given receipt of that even he has show the less value of the flat on agreement then actual paid by me.