GPA expired due to gpa Holder demise.. transfer of property to the gpa holder's legal heir

Good Evening Lawyers. My father had bought a land property in 1998 from a friend who is the seller for say Rs 100. The GPA was done and it was notarized.(Please note that the GPA has not been registered due to their friendship and understanding) The amount transaction for the transfer of the property has been completed then itself. My father constructed the house and we have been living in this house even today. My Father expired in 2018, since the GPA holder is expired, the gpa fails. However, The seller is ready to register the property to the legal heir... But since the property has grown to around Rs.6000 today the seller says he will get capital gain around RS 1500 and is not ready to bare it , and asking us to bare it. (Where we do not have a choice to deny) Question: 1. Since the property's amount transaction is already over then, can we register the property based on the previous transaction mentioning about the gpa ? Will this transaction be valid? 2. If the above is valid, will the land owner get the capital gain for today's value? When there is no amount transaction being done today , In what way will the seller get the capital gain? 3. When we are not paying today's SR value to the seller, will the transaction become valid based on previous GPA. Can this deed be made and will this become a valid transfer of the property? 3. If the above transaction is not valid then what can we do.. Condition: 1.Here we are not making any transaction with respect to the present property value, to the seller today since the transaction is already over. 2. The property has to be registered with the stamp duty paid on the legal heirs name. Please give us the exact legal advice. Thank you