Cheated by builder

In July 2015, I booked a plot with a builder in Bangalore by paying a deposit of ₹1 lakh. Over the following year, their sales executive repeatedly pressured me to make the full payment, which I completed through multiple transactions by August 2016. Since then, I’ve been consistently following up and requesting registration of the plot, but the builder’s team neither responded nor provided any updates. After more than 8 years of follow-ups, they finally informed me that, since the registration was not completed, the plot had been sold to someone else. How is this even possible? Now, when I ask for a refund, they are offering only the principal amount without any interest. What should I do in this situation?