My Uncle Deceived Me !!

Hi, There is a piece of land around 3 acres in survey nos 228/1 and 228/2 in which a patta is in my father's name. My paternal uncles have their respective pattas in the same land without demarcation. We have his passbook of the same and I along with my mother, two younger brothers and two sisters inherited my fathers share from his patta land. One of my paternal uncle who lives in the same place where land is located offered us 50 lakhs to buy our piece of land. We created an agreement of sale on bond of 100 rupees. He paid us in installments a total of 45 lakh rupees and convinced us by promise that he would pay the rest 5 lakhs in 8 months. He took my signatures on receipts/papers mentioning that he paid 50 lakhs in total. Now, he says that he paid the amount in full and denies that he has to pay anything to us. Fortunately, non of the legal heirs of my father were involved in this except me and he took signatures ONLY from me since I am the eldest and my two sisters and one brother are still minor. Now my question is: 1. How can we recover our land since he has constructed shops in our area and have already sold them to third parties. 2. Do we have to file a partition suit to get our share of the property back? 3. Can I challenge him in the court of law in any means? 4. If yes then how can I stop him from selling the property without filing a partition suit. Please Note: There was only an agreement of sale between me and my uncle and no registry or mutation was ever done in revenue records. The property still appears in my fathers name in the records.