Original agreement to sale is lost

You can get a certified copy of the registered sale deed from the Registrar's office. It is not advisable to purchase a property without copies of chain of documents. 

Contact a local advocate and take his guidance in publishing the facts that the seller has stated that original lease deed, a complaint has been registered with the police, there were no encumbrances in the Sub Registrar office, a mutation was in the name of seller only, the society records also showed that the seller is in possession, and hence as the seller is providing certified copy (Let the seller obtain such copy in his name) of sale deed, the purchaser intends to purchase if no one objects.  Wait for a reasonable time and act as per the guidance of the local advocate.

Thanks for your responses.

 The original sale deed is present and not lost.The original sale agreement is lost.

you better take help from landsdoctor. app. This app provides land property genuinity.

To my understanding, the missing or lost agreement is not of any consequence.

Absolutely true.  The agreement to sell is not important after the execution of a regular sale deed.  Even if the court executes the specific performance decree, they have to destruct the agreement to sell .  It is just a supporting document till the regular sale deed is executed.