1. you should have directly paid the 70 lacs to the lender bank of your seller instead to the seller
2. have you received possession of the flat?
3. you will have to file a suit for specific performance and take out interim application for urgent reliefs
4. if you can arrange the dues of the lender bank of the seller through any source, then consider paying those dues and obtain a foreclosure letter from that bank - atleast this will save your flat from being auctioned by that bank to recover its dues
5. then you will have to agitate your claim against the seller