Stamp duty on construction agreement

Hello, FACTS: I hold a commercial property in Ahmedabad. It was purchased from the builder by making a sale deed of undivided share of land and construction agreement in the year 2005. Stamp duty is paid on the sale deed for the undivided share of land but not on the construction agreement as it was a trend during that period. UNDERSTANDING: as per my knowledge it was not illegal then. After that, government made some amendment and stamp duty on construction agreement was also made compulsory. PROBLEM: Now as I want to get education loan from SBI by giving the same property as collateral, SBI is asking to pay stamp duty on construction agreement. Is it actually required? If yes, please share the link of such circular which enforces paying of stamp duty and how much stamp duty to be paid? If no then How can I convince SBI people to accept the property as collateral without paying stamp duty? If possible please share the amendment which was made after 2005 making the stamp duty compulsory for construction agreement. Thanks in advance. Varun