How to proceed if buyer not able to make full payment of property.

In brief ,I sell my property to one person and he paid only 20% money and prepare a agreement that he will pay rest money in seven month but now he is not able to pay the money and requesting for additional time … additional time of 3 month are finish still he is not able to pay rest of the money …

One problem is in first agreement the name of buyer was not mention because he told me he want to make agreement his cousin and I leave blank on his faith.

Please advice.