since he is your closed relative in my opinion power of attorney with right to sale should not attract stampduty as on a sale deed. i am from up and a koltta person would be fit one to speak as land legislations are in state list of constitution,states are free to legislate their own way. and mostly no uniformity is found among different legislations.However,GIFT is stamped as a saled deed so no scape there.
One idea may prevail for relenquishment/surrender that your nephew files a suit claiming his exclusive right over property in question either claiming relenquishment or surrender or adverse possession or even via will from your mother(whichever option possible there from these3) and serves a notice of suit via court on you where after you put appearence in the court and admit his suit claim then a decree shall be passed in his favour by the court that he is ex clusive owner of the property to the exclusion of yours' and then he would be able to sale it alone and no tax incidence will be attracted to you.But certainly you will have to visite once there in kolkota court to admit his exclusive right setup in suit about property under discussion..