Cancelling a sale deed due to issues with property paper

I have a property that has been divided amongst my father and his 3 brothers but that division was done mutual and not on paper. I have the power of attorney but though it confirms the amount of area I own but not the direction. I have the posession of my part of this land but this hasn't been described in papers. I came it a sale deed with a party intrested that includes a 6 months payment plan with me providing assistance in paper work. This has been quite tricky as I found out without agreement of the other 3 owners of this full land I cannot get a survey done to get my papaers corrected. I advised the buyer that I wouldn't pursue with the deed as this will take time anf I dont wish a family chaos. He advised me to get the registry done under his name and he will get a back door entry and acquire hold on this property. I being suspecious of his unlawfull intentions clearly ask to cancel the deed but he denied. Now he has sent me a notice from an advocate to acknowledge the sale deed accusing me of running into another party for a higher offer and not honouring his. This sale deed ecpires in 2 months but the notice states he would file a case in 15 days. What are the cosequences and how do I put an end to his bullying and this deed.