Dispute with Migsun Vilaasa

In 2018, I reserved a flat at Migsun Vilaasa in Greater Noida. At the time of booking, Migsun presented two options: 1. Have the interiors completed by the builder for an additional cost. 2. Select a bare shell unit, where Migsun would complete only the exterior structure, leaving the interior work to me upon handover. I chose the bare shell unit to have the freedom to design the interiors according to my preferences. This choice is documented in the Builder Buyer Agreement (BBA). In December 2022, I received a final demand notice for ₹13,57,951, which exceeded the agreed amount of ₹11,04,250 specified in the BBA. Migsun provided this inflated demand without any explanation for the additional charges. I responded to their email requesting clarification on the increased demand and received a phone call in reply. During the call, Migsun claimed that a new government regulation mandates that builders complete interior work. However, when I requested written details of this new rule, they instructed me to search for it online myself. My attempts to find such a regulation were unsuccessful. Migsun continued to communicate this information verbally or via WhatsApp from various unverified numbers. I insisted on receiving written confirmation on official letterhead or through the company's email address, but they refused. By April 2023, Migsun had not provided details of the alleged new rule but did share a list of interior work corresponding to the surplus amount demanded. At this point, I had agreed to let Migsun handle the interior work and was prepared to make the payment, this is when I received another notice in from them demanding an additional ₹2,05,965 as a late payment fee (from dec'22 to apr'23). Since then, I have repeatedly requested the removal of these extra charges, but they insist that I must pay the full amount before any discussion can take place. They remain unwilling to negotiate.