Unregistered stamp paper of property

We bought an agricultural land from my cousin uncle for Rs.150000 in may 2005 , as the land was under 32G it was not transferrable at that time , so we made a agreement on ₹100 stamp paper on my wifes name stating that he received ₹150000 and the land is sold but the stamp paper was not registered or notarized ,the stamp paper had 5 witnesses in which 2 were cousin uncles son , 1 was myself , 1 was my real uncle and 1 was from village member but he is no more , the title on agreement is Promise letter. Since 15 years the claim of land was with us but now his son's are claiming back his land saying that the stamp paper is not valid. 1) Is this Stamp paper valid? 2) is there any law which says that stamp paper should be registered? can you please help with this Act no.? from which date or Year this Act says that all the stamp papers Should be registered? this stamp paper is near about 15 years old. can we put case on my Uncle on the basis of this Stamp paper? Can we atleast put a stay on claiming the land for atleast 10-15 yrs , i dont want to give him back the land so easily. my uncle start disturbing us with considering that this stamp paper is unregistered so it is having "Zero" value in court.