1) it is necessary ti peruse agreement signed by you to advise
2) since it is govt land and no construction is permissible terminate the agreement
3) you can issue legal notice to your friend family to recover amount spent by you on construction and For refund of Rs 15 lakhs paid by you
4) if they refuse to pay then file suit for damages ti recover loss caused to you
5) also file police complaint under section 496 ,420 of IPC for criminal breach of trust and cheating