Land dispute

I bought a land of 132 sq yards in my home town. This land is sold to me by Mrs A in 2015. Mrs A bought it from Mr B in the year 1992. Mr B bought it from Mr C in the year 1990.I have all the link documents. Got permission from local authority to build. Constructed a shop and residential portion. I got house number and paying tax as well. Now the sons of Mr C put a case telling that without their knowledge Mr A got the sale agreement done from Mr C. They have completely ignored that it is sold to Mr B and not Mrs A. The only proof they kept is PAHANI copy where the entire land entry is showing 1.01 Acr and current status of land as ABADI/Houses. Just my attaching pahani copies of multiple years they are claiming their right. Since we got house building permission and house no we thought land is not agricultural. Now a case has been filed . Can some one please guide me on how Municipality gave permission if it is still agricultural land. They have no passbook only pahani entry. The lawyer now demanding 5 Lacs to with draw the case.The case is put by him as the plaintiffs are not very educated Can some claim right over the land that is sold to 3 different hands. What is advise to sue the lawyer and documents i can produce to the court. Thanks