Buying a agriculture land in sqft value

Hi, I am planning to buy a agriculture land of 53 cents with a well and coconut trees, which in its current survey mentioned as plot land and its valuation comes as 67 rupees/sqft. Whereas nearby locations agriculture lands are valued as 8 lakhs per acre. Because of this there is a huge difference in stamp duty and I am more worried about missing the benefits which I get as a small/marginal farmer. The current owner when registering has done a undervalue registration which proceeded to 47A and by a stamp registrar the value of registration came to 40000 Rupees. I am being told by writer office and registrar office since it was already registered once with stamp registrar, I cannot do the same way and I need to register in sqft valuation only. Is there a way i can retain this as agriculture land to avoid huge stamp fees and also i can be eligible for the benefits of being a small/marginal farmer. I am not able to get a procedure to convert residential land to agricultural land. Your response will be very much helpful. Thank you.