it is necessary to peruse agreement signed by you with buyer to advice
2) there must be a default clause in the agreement . it would mention what would the consequences in case purchaser defaults in making balance payment
3) contact a local lawyer
4) issue legal notice to buyer to pay the balance amount within period of 7 days from date of receipt of agreement
5) i presume you have not delivered possession of the flat to purchaser
6) you have to file suit for specificperformance