Revoking an irrevocable power of attorney

My mother, Swapna Sarkar, age 66 is owner of a property measuring 2 katha 10 lechas in Golaghat assam. This property is my father's property and not an ancestral property. My dad's elder brother's family is living on it since the plot of land was bought by my father around 1977.This time around in 2016 due to our financial constraints, I and my mother is willing to sell the half of this land. But my cousin brother Mr. Habul Sarkar is had some conspiracies to withhold this sale. To do this, he all of a sudden writes objection to our sale permission which strangely is entertained by the town sub district circle officer. Due to the fact that we do not stay in Assam and want to quickly finish this deal and go back to our native in Gujarat,we got immensely psychologically pressured by him, lost our judgement for moment and through his coercing, signed a 100 rupees stamp paper notarized and labeled " Irrevocable Power of Attorney" for his half of land that he is staying right now. and now he is not letting me sell my half by illegal occupancy. My question is that I want to say again that this " Irrevocable Power of Attorney" does not have clause anywhere inside the document's paragraghs or clauses that " My mother has irrevocably appointed Mr. Habul Sarkar/this power of attorney is irrevocable" and also this document is not registered with sub-registrar as my mother did not go to sub-registrar's office with him to complete this procedure as he is not in knowledge of need of complete registration. Does my mother need to revoke this or if she does, what is the proper way--- a simple notice to the agent i.e. Habul (he may not return the document) or publishing the same in local publication. Please tell me a way other than a legal case as I don't have time or money to invest for a long time as I do not live here.