Sell of ancestral agriculture land

We have ancestral land in one of the most prominent parts of Gujarat. After my father died we were coerced into giving POA to my uncle who lives in india. We live abroad. The land is in form of survey numbers. Some survey numbers is owned by my father and some of them by entire extended family. My uncle has now informed us that the land is sold and the buys is raising cheques as per title on surevey numbers. However before the cheques can be released to us, my uncle has asked to return 42% upfront. i.e. give him upfront chqs in the name of some company before he can release our cheques. The explanation I was given is that the land was sold less than the 'Jantri' price but for official reason the seller cannot use actual low price. For e.g. Jantri price is say Rs100, the deal was made for 58 and so we have to write upfront chqs for 42 before we get 100. As the seller cannot officially show 58 as the sell price. And now he is also asking us to sign a legal document saying we have received our share and in future there will be no claim made. I have no way to know what is going on or who to contact. Since I was told it is a very costly matter to get legal help and I have be there physically. Any help is appreciated