Misleading registration

Hello sir, Myself Krishna. I have 3 elder brothers. My father bought a property of 353 sq.yards(7.30 cents), from Housing society in which he built a 2 floor building in around 4.8cents/210sq.yards(approx). He has written a Will saying that me and my 2nd brother should take the house building and remaining vacand land should be divided between two brothers. Our plot having a cross shape and besides of that unsold private land belongs to housing society. After his death we ((son 2&4 reffered as Party1) (son 1&3 referred as party2)) both parties went to document writer and signed a partition deed. In partition deed the document writer added the private land also and made a partition like 235 sq.yards to Party 1 and 237 sq.yards to party2. We didn't make any objection as we Party1 received our own bit and as well that Private land is not coming under our part of Partition. Note: Partition deed not registered, it's only executed on 100/- bond paper. Later after few months Party2 gave Power of attorney to Document writer for selling their partition. Then he sold to a party by registering 239.6 sq.yd by giving a link document number of original document(my father's 353sq.yd document). We didn't register our property. Now we, Party1 want to sell that house, for that we are getting objection from buyer, saying that we can sell only 114sq.yd ( 353-239) as Party2 already sold 239 sq.yd to someone. What can we do now. Kindly suggest possible way to solve this issue.