Property sold under a Sale agreement

I have sold my flat at Kolkata under a sale agreement executed between me and the purchaser wherein clause 3 of the agreement states that "3. The Purchaser has paid to the Vendor an amount to 75% of the total price as initial amount which the Vendor does hereby acknowledge and confirm towards earnest money after which the possession of the flat has been handed over by the Vendor to the Purchaser. The balance amount would be paid to the Vendor on or before TWO YEARS from the date of signing this agreement with an option that the Purchaser can pay any lump sum amount to the Vendor at any point in time subject to a minimum amount of Rs. 7,000 (Rupees Seven Thousands only) per month payable by the first week of the English Calendar month which will be treated as rent per month till such time the full amount as agreed upon is completed when the same will be adjusted with total agreed amount as paid. " The purchaser also assured orally along with the witness (which was endorsed in writing afterwards) that he will complete the payment of balance amount within six months and execute the sale deed. But now he is neither paying any monthly amount nor executing the deed by paying the balance amount. Wanted to know what legal action if any can be initiated against the purchaser.