Builder using sub standard material in a house completion

We are constructing a house in Chennai through a builder. we did not sign any mutual agreement. Initially verbally we mutually agreed that the construction would be done on the basis of Rs.1600/sqft for ground floor and Rs1300/sqft for first floor inclusive of septic tank & Compound wall. The Preliminary estimate stood at Rs.50lac and the construction time would be around 3months in the due course of construction we also increased the construction area in the first floor with an extra room making the overall to be around 57lac After 9months now, we have given 45Lac so far in small installments (recorded with the builder signature in a hand written notebook). The House is now 80% complete, we now find their quality of woodwork and granite to be unsatisfactory and asked them quit the construction and complete the complete the work to the extent of the amount paid so that we can finish the remaining with some one else. The Builder is now refusing to quit and asking us to pay the Rs.1600/sqft for ground floor and first floor with septic tank & compound wall extra charges, amounting to about 75lac (instead of the agreed estimate 57lacs) The Builder also threatened us that he will lock our construction site and will not allow anyone else to work on our site Work is now halted at our site. kindly suggest how i can make the builder to complete the work for the amount paid 45 alone and disengage from him, so that i can finish the remaining 20% with some one else.