Calculate your gain in rupees, then convert that amount into US$ using the exchange rate as of the date of sale.
Your 2020 economic profit is determined by your net profit in rupees as of the date of sale (in 2020). That is the only amount you will have available to pay tax in US$.
Gain = Sales proceeds in 2020 rupees - Cost in 2006 rupees - cost of improvements in rupees (at time of improvements).
