Pagdi symtem regarding after father die.

Hi, My dad had bought a pagdi property in mumbai from a landlord by paying huge amount on money. Unfortunately, he passed away in 2006 and has left us all in shock. We spoke to the landlord to ask him to change the name from my dad to my mother name on the rent receipts. The issue is that the landlord is now asking us a sum in lakhs to transfer the rent receipt on my mother name. This is really ridiculous as we are not selling the property and we are only transferring the name since my dad is no longer there. I wish to know on these things: 1. Is it right on the landlords part to ask for such exorbitant amount of money for transfer after death ? 2. The property is currently sub-let out to another person who is paying us monthly rent out of which we give 10% to the landlord. The landlord has also signed the sub-rent out agreement and the agreement is notarized. I want to know will this create a problem if we take court's help to transfer the rent receipt on my mother name. (is it a problem if pagdi property is rented out by the tenant?) 3. We have only father name of 18 year old rent receipts and Electricity bill & Ration card. Please help me with these queries.