Interest charged without releasing demand letter

Hi Team, I bought a commercial property in 2013. I received two demand letters from the builder and made payment in timely manner. Post that I haven't received any demand letters from the builder. I went to builder's office couple of times and got to know that there is an amount of Rs 10000 has been charged on my account towards interest at 18%. I made the payment in a good faith as staff promised to look into it and next time same incident happened again. After making two payments in good faith and broken promises from the builder, I stopped making the payment towards the principal amount and the interest already charged. I requested the concerned person to set up a meeting with the higher officials of the builder lot many times but they did not allow me to meet them (Communication happened over mail and phone). Now, property is nearing to possession. If I make a payment towards the principal amount can the builder stop offering the possession as I am not going to pay the interest amount. And is it possible that I can file a case against the builder to waive of the interest charges and compensate for the mental harassment caused in the entire case,