Property owner cheated by selling already sold property, What actions we can take against this

My mother bought a in Bangalore in June 2015, the person who sold out the site has already sold out the site to somebody else about 15-20 years back, before Registration only he has collected 500000/- from my mother and at the time of registration he took 500000/- he told all the documents are with his lawyer for some other issues and he will give it to my mother later, he himself brought the registration paper printed & told he himself will pay the registration fee & stamp duty, for that he collected 50000 extra from us but he mentioned only 550000/- in the paper, but I saw it later after registration, when I asked about that he told they will mention only the government value, since his sister's daughter marriage was there he told he need money immediately & collected balance amount of 450000, later when we asked for the papers he told you had taken EC that's enough, we will give Balance documents once our case is cleared we will give, later when we went to construct a house in feb 2017, some other person came claiming that site belongs to him, that time the person who sold the site told us he is showing the duplicate documents don't worry I am out of station I will lodge complaint against him, later he told he is filing case against that party, that party started construction of house at that time also the person who sold the site told us if he construct the house in your house will it becomes his, don't worry I have already filled case, but we keep on asking for the complaint copy, case details but he keeps on telling it's with his lawyer he will give tomorrow day after tomorrow etc., For about 3-6 months, since my mother had health problems we couldn't go that side, later we complained to Bescom where the person took meter connection, but they shown us the documents given by that person then we came to know that the site was sold before only to some other, now he just keep on telling his case is in court he will give that site only to us, if not possible give other sit, he is not giving any case filed documents each & every time when we ask he is just postponing saying that it will clear in one week, one month, 3 months, 6 months etc., When my mother said something like we will give complaint, he is threatening saying that if you give complaint you will not get the site and not your money back, I know how to come out of this like that, when I myself asked to me also he was threatening, what steps we can take further, please advice