Agreement to sell will not prevail over the registered sale deed. Because the agreement to sell does not confer any right to its holder as per the provision of Section 54 of the Transfer of Property Act. Therefore, the holder of agreement to sell cannot seek cancellation of sale deed.
You have accrued right in the property through the registered sale deed. If the holder of an agreement to sell is trying to assil your right, you should file a civil suit for declaration and permanent injunction. For the declaration of your right in the property you should file a civil suit under Section 34 of the Specific Relief Act 1963.
In the declaratory suit you should make the seller and holder of that agreement to sell as defendants. You have a valid sale deed then the court shall declare your right in the declaratory suit and preclude the defendant to assail your right in that property.