My Labour Contractor Is Harrasing And Demanding More Money

I had hired a mistri for my home construction of G+2. Approximate cost as per rate discussed comes out to be around 13 Lakhs.The work is not yet 50 % complete and mistri has already taken around 9 Lakhs. With his calculation, he is telling me to give me 70 K more for the work done. But the rate that he is putting for each work is very high then the market. Like brick work for 50-/ and more. I have started getting cracks in the work done as well as leekages in the tanks. He has been demanding money and warning me that if we don't give him then he won't start the work and will involve more people who will help in his settlement. As per my guess, he might not come once even if I give him money. he is just delaying my work. It's been already 1.5 months. Request your expert inputs and help here.