mental harrasment and threat on phone call

You asked the same query by moderate manner now. Based on your statement your father had some connection with the person. You firmed that the person not having enough proof regarding paid amount then you can complaint to police against his torture . You better to consult a local lawyer and act accordingly 

There is no clarity on the issue. 

However, the legal heir or any citizen can provide an original death certificate to Sub Registrar and seek a certified copy of such will.. With costs less than Rs.200/+ you can get a certified copy of will.

First, the property should be transferred to the person named in the will, and then only he is having authority to further transfer if the property is in his name.  Further, when some court cases, GPA, etc were there the proper way, is to contact the advocate for knowing the facts.  ( Your query: he first said GPA, will, court order, unregistered sale agreement, he possess original documents, etc., he also demanded 9L money, else he will sell property.. he is showing incomplete documents)

Record the conversations and file a complaint to cybercrime that you have been receiving threatening calls from so and number and ask them to record the conversation officially at your request for proceeding further in filing a regular complaint.
 

With the available documents and records, please approach a local able and reliable Lawyer and explain the matter cogently and seek his professional help.

The facts posted are disjointed and lacks clarity.

He cannot sell the property on the basis of an unregistered GPA.

Further the GPA becomes invalid or automatically cancelled after the death of the Principal, hence the power agent, even if possess the registered GPA cannot sell the property after the death of the Principal.

If he states that he has a Will, then to enforce the Will, he needs NOC from the legal heirs of the deceased testator, at that time , you can refuse to give NOC, let him approach court for probate of Will, where you can challenge the Will if it was obtained by playing fraud.

If he states that he has a court order, let him execute the court order because again in the execution petition he has to implead the legal heirs of the deceased judgment debtor, if at all he has a decree in his favor.

You can refuse to pay him even a single penny.

Further you need not search for a copy of the Will, if he relies upon the Will, let him go to the court of law seeking relief, you can challenge the same on merits and documentary evidences in your support.

In the meantime you my file an injunction to restrain him from alienating or encumbering the property in any manner. 

You Can take the assistance or help from  local advocate to tackle the issue legally.

Certain facts posted are vague and incomplete, which can not help to form proper opinion and oblige on the basis of presumptions.

Consult and engage a local prudent lawyer for appreciation of facts/ documents, professional advise and necessary proceeding.