Promissory Note, Blank documents, High

I am a Freelance Interior Designer. In 2012, I ran into a huge loss to the tune of 15 lakhs. To overcome the loss and come out of all the commitments, I approached a Person who said he will help me by giving a cash loan of 15 lakhs and also partner with me and further invest another 15 lakhs to create a proper infrastructure. This person gave me a total of 15 lakhs as promised. (Please note that the money was not given to me directly as lump sum, instead was paid directly as cash for Material and for workers to finish pending works). He took two cheques 7.5 lakhs each without date, a promissory note for 15 lakhs and a blank stamp paper with my signature and thumb impression. Later he did not invest any amount nor create any infrastructure, but we started taking new projects and he was adjusting my share of profit towards repayment. This is continuing for last three years and all my earnings as profit are going towards repayment. I am hardly getting any money even to take care of my basic survival. He has already recovered more than the amount he lent me from the profits. Approx around 20 lakhs. You must be wondering as to why I have been working free all this while. From order to order he tells me that we will settle accounts and start a new Company. I have been very patient and now clearly understood his delay tactics. He is also not giving any acknowledgement/receipt that money is being adjusted from my Profits. Now when I ask him for receipt of money paid till now (or) Tell him that I want to start working on my own, He is calculating very high rate of compound interest and asking me to pay another 15 lakhs and also harassing me with messages and phone calls. I am asking him to return my documents and cheques which I gave him while taking the loan. I do not want to work together with him anymore. How can I come out of this? What misuse can he make with those documents? Are they valid anymore? Should I initiate any legal step first? Please help me out with a solution. Thank you. Hari Shankar