for buying a resale flat from a person who has power of attorney

We are about to finalize a flat (resale) in Sukhobrishti (Shapoorji Pallonji complex) in Rajarhat , Action area III. The flat has been handed over to the owner about 6-7 months back and it is an unused flat till now. The owner of the flat has given power of attorney to his brother for selling the flat. Owner's brother has shown us the flat and he has shown us the original conveyance deed and the power of attorney papers. We have not yet met the owner of the flat but have met his brother who has shown us the flat and other documents. Now our query is , whether it is legal to sell a flat with power of attorney? In that case do we have to make the payment for the flat to the owner's brother who has the power of attorney? We would like to consult a good property lawyer for this purpose. Also before paying any booking amount/token amount we would like to get all the documents of the resale flat , verified by an experienced property lawyer. Kindly let us know how to proceed with it. Awaiting your early reply. Thanks & Regards, Dr. Sayanti Das. Uluberia, Howrah.