Land seller not giving the posession after registration of land

My Friend had purchased a land and registered under his name but the seller is not ready to give position to the buyer. Now it is more than 2 years passed every time we ask he ask for giving the position fo land he always make some excuse and ask for some time But Now he says i will not give you position take back the money you had give, he says it was my mistake to sell the land noe he want it back as the value of land is appreciated and also he says he wand to construct his houe for which he also need this land also Actually the owner had land from which he sold to my friend a part of land Kindly sujjest waiting for your reply