Not giving possession

Dear Sir, I would like to ask you wheather builder can sell my flat to some one else without giving me posession for the said property after being paid total amount? My second question is that I have paid all the amount to buider in Feb 2015 for the flat whose posession was supposed to give in 2013 but due to some reason buider was nt able to give. He sent us a final payment letter in Jan 2015 & we made payment in Feb 2015. When we went to buider office to make final payment, his sales people started saying that they can not give us posession cos they have to give this flat to PWD department. When we asked them for the written proof from PWD they refused to give. When we asked them that we will give our flat to them after giving the written notice that they are going to give this flat to PWD department, they again refused to do that. Few days ago we visited their head office, their head of sales department said that they have sold our flat. So is builder liglly allow to do so without our permission? What next step shal I take. Please help me.