What legal actions can a seller take for an unclosed agreement?

An unregistered agreement of sale was made for a property transaction worth one crore. Two buyers jointly signed the agreement with the seller. They paid Rs. 20 lakhs as an advance, and the balance was to be paid in six months. However, after four years, the buyers haven't paid the balance and haven't closed the contract. The seller has reminded them several times to settle the balance. The seller is now ready to refund the advance after deducting interest and penalties. What legal options does the seller have to close the agreement? What are the standard terms for deducting penalties and interest? Can the seller sell the property to another buyer who is ready with payment?