1) In general, litigant buyers always ensure to consciously avoid mentioning
a) time line for payment of remainder money or
b) time line for registration..
So it is not surprising that the buyer did not mention the same in agreement to sell.
2) We presume that your agreement to sell is unregistered.
3) If the agreement to sell is unregistered, then you are advised to issue legal notice to the buyer asking him to come forward for payment of remainder of monies and also for registration.
4) If the buyer does not respond to your legal notice, then you can unilaterally cancel the agreement within 30 days from the date of receipt of legal notice by buyer.
5) If the agreement to sell is registered, you will still need to issue the legal notice, but in order to cancel the agreement to sell, you should file a suit in the court with a prayer to cancel the sale deed as registered agreement to sell can only be cancelled through court.