Buy property from Bangladeshi National in India

A Bangladeshi national got a flat in Kolkata as inheritance on 2010. He gifted the flat to his son who is also Bangladeshi in 2015 by registered deed. The son gave power of attorney to a Indian national by registered deed. No mutation is done by father or son as they never stayed in the said flat. I am an Indian resident and staying on the flat since 9 years on rent. Since last five years no new lease and leave agreement is done. For first five years my lease and leave agreement was done with the power holder and money receipt was given. Now every month I am paying the rent in cash to the power holder without. any receipt. Now I want to buy the flat legally. The Indian power holder is saying that according to latest regiatration laws of West Bengal the seller must have indian pan and aadhar to sell the property and get it registered in the buyer's name. So in this case is the Bangladeshi person will never be able to sell his property? What are the restrictions and what is the procedure? There must be a way. What can I do to buy the flat with home loan from a reputed bank?