Sale agreement of land.

My father was leaving in a block and was leaving with my mother . Myself and two sisters were staying at other place.My father expired on 01.12.2022. Now the land and other properties has been transferred in the name of my mother , myself and my two sisters. There is one valuable land in the market measuring 12.5 decimal and it's present value is around 5 Cr. In this land there was a tenant and was running a hotel by constructing a small shed. When all of us decided to sell the above land after 15 months of death my father then tenant shows us an agreement on 01.03.2024 executed for sale with my father on 1000 Rs Stamp paper. The agreement was executed on 10.07.2022. The agreement states that the purchaser has given Rs 25 lakh in cash. The total sale value in agreement is 1.40 Cr. When the agreement was executed my father age was 84 and was running with lot of health issues. Validity of this agreement is upto end of 2026. The agreement is signed by purchaser and my father , two witnesses and one local lawyers. This agreement is not registered nor notarized. Kindly advise 1. Is this type of sale agreement by paying Rs 25 lakh in cash is valid. 2. Stamp paper duty of Rs 1000 is sufficient for execution of Rs 1.40 Cr value property. 3. Is registration in court is required or not for this agreement. 4. Can the validity of sale agreement be 4.5 years. Pl advise Bipin Kumar