1. If the entire sale consideration amount has been paid and there is a mention in the sale deed, then the seller cannot claim the same again on the basis of the sale agreement. Once a registered sale deed has been executed then the sale agreement becomes infructuous.
2. He cannot file any suit on the basis of the photo copy even if he is stating that the original is held by the opposite party because the sale agreement itself is an unregistered document. Moreover he cannot suppress the information that the property has been sold vide a registered sale deed, hence just wait and watch the developments.
3. Until seen what is written on it and what kind of book that is, no opinion can be rendered on it.
4. Just wait and watch the development, if he is filing a suit then you can file a petition to reject the plaint under order VII rule 11 for the reasons you may rely upon.