Sell of land

A registered deed of agreement was made with seller for 1 katha land, but the very next day he called me to inform that he will have to pay a huge amount as TAX on the amount he will receive by selling the property for which he told me to sign in a white paper to cancel the agreement by morally blackmailing me telling that he will die of high pressure stroke as he is a diabetic patient with hypertension. Looking at his helpless condition, i signed the paper written by his advocate son-in-law. After that both of them proposed to me that they are going to sell the plot of land to me only, provided I give them some amount as cash as per their demand so that at the time of sell they will hide that amount received and eventually avoid paying tax. As per their demand I gave them Rs 11 Laks duly signed in revenue stamp written as advance amount for sale of that particular plot of land as mentioned in the registered agreement. Now it is 3 year I paid them money, but they are showing their unwillingness to hand over the plot to me telling that you cancelled the agreement, so we are not going to give you the plot. And also not willing to return the money that I have given to them for which I have already paid Laks of interest. The seller tells that slowly he will return the money as he is now retired!!! And now he is forcing me to legally cancel the document within 7 days. I kindly beg your opinion what should I do. 1. Can a court registered deed of agreement be cancelled in a white paper? Or it should be done in court? 2. Can the seller take money from buyer for a property after cancelling a sell agreement of the same in white paper? 3. If I go for legal help, is there possibly for me to get the property after paying the remaining amont as per agreement? 4. How many years it may take? 5 . Can phone call records be used legally as a proof of discussion? Please help. Kind regards...