Co-Ownership Nightmare: The Tale of Two Tenants

Rahul and Karan jointly bought a flat through a Registered Sale Deed. The property is 500 sq. ft. and the registered sale deed states a 50:50 share of ownership. After 2 years, they both decide to sell their respective shares. Rahul wants to sell his share to a third party for ₹1 crores, while Karan wants to use his share as a mortgage for a ₹75 lakhs loan. But here's the catch - Karan had made some significant modifications to the property without Rahul's consent, and Rahul claims these modifications have reduced the resale value of his share. Can Rahul prevent Karan from selling or mortgaging his share?