Builder demanding flat cancellation charges at 10% amount is it legal

Hi, I completed application form to book apartment with leading builder (Aparna Constructions) in Hyderabad and paid initial 1 Lakhs rupees around 2 years back. I changed my mind and didn't paid 20% required for booking completion or to proceed with Sales agreement. I clearly communicated over email my intention to not proceed and requested refund of Rs 1 Lakh; however they did not respond or refunded my money. Now after almost over 1.5 years; I got letter from them regarding cancellation of my booking and they are demanding 10% of apartment cost as cancellation charge which was under some clause in booking application. I need advice about next steps; I did't entered in Sales agreement or paid 20% to confirm booking next 30 days of application signing so what is my liability especially after over year based on plain application form (it is no legal stamped doc). -DA