Related to undivided property from our grand father

Dear Lawyer, I am happy to see this site where the lawyers are volunteering to give legal advise. I appreciate your advise and thanks in advance. Our grand father has a property of 6 acres which is not partitioned. My grand father has three sons on whose names the pattedar passbook with 2 acres on each name are issued. On a request from our father first brother we allowed him to sell 1.5 acre for his daughter marriage and this land is deducted from his pattedar passbook. Same request came from father second brother to sell 1 acre, we accepted his request based on 2 acres will be given together at one place to us. This discussion was verbally agreed and made an agreement with the seller for 1acre. Later, with out our acceptance, the purchaser made registration of the one acre land as plots from the second brother with a dimension mentioning the neighboring land 2 acres belongs to my father. In this document, only the second brother family is mentioned as sellers. The first brother also signed as witness. My father hasn't signed this registration document. Now is this dimension agreement valid, Can we go with construction in this 2 acres? When we requested my father brothers for the partition document, they are going away from their verbal words. We have the right to stop the work going in 1 acre purchased, but purchaser says, my father shouldn't have problem as two brothers have already agreed that 2 acres is my father's land in the dimension. We don't want to trouble others, but at the same we don't want to loose. Please advise. Thanks in advance for your legal advise.