Cancellation of sadakat

I am an Nri, we have ancestral land in Surat india that has been usurped and built on illegally, I have all original deeds and documents and have the propety registered in mine and my family members names. I have been a approached by a so called builder who tells me that they will purchase the property with these issues and that I don't have to have a long drawn out court battle. This builder party has asked me to sign a sadakat document which they say is just a means to show the parties that have usurped our land that they are going to be taking over the property and these people will have no option to vacate. No actual amount or value on the property has been agreed upon between myself and the builders and there is a letter to prove this, however this sadakat document has an amount of 50 lakh reflecting and a token of 5 lakh is supposed to be received by me, I have a cheque that has been given to me buy the builders but I have not cashed it as I'm not happy about this arrangement Also I am been asked to give a power of authority on the builders name I'm not certain I should do this Is there anyway that I can have this sadakat cancelled and this cheque returned, this document has been stamped by their notary, Also if I can do I need to come to India to do the cancellation or can it be done from out of India, your assistance in this regard will be highly appreciated