Dispute for rented shop

Hi Sir/Ma'am, I have a rented shop for more than 60 Years now. Area of shop is around 500 sq feet and we are paying rent of 700 Rs per month. Rent is low because we have been staying on that property for very long (since my grandparent's time) and its very old constructions ( tin shed roof and old bricks clay walls). We never have had any rent agreement for that property. Now owner suddenly is asking for 7000-8000 Rs per month rent else asking to vacate property. We asked owner to sell it to us but he is quoting very high price than the current market price. As per current circle rate shop's value is around 25 Lacs but he is asking for 60 Lacs. We are ready to pay around 40 Lacs but he is not agreeing on it. Although we pay rent monthly but owner used to give us yearly rent receipt and we have receipt for around last 30 years. Since last 11 months he is not accepting rent, hence we don't have receipt for last 11 months. All rent receipt which we have is on name of my grandfather who passed around 20 years back. Request you to please assist me on this case as this shop is only source of income for my parents and uncle family. Suggest me what action we should take in this situation. Also how can we negotiate for buying this shop as we want to do construction on this property. Thanks in advance for your help and guidance. Regards, Sumit