Differential amount in sale deed and agreement of sale

Hi Experts, I Have entered into Regd agreement of sale to purchase a residential site in karnataka from vendor X for a total Sale amount of Rs. 45 lakhs paying 15 Lakhs in advance, with an agreement period of 3months to complete Sale deed formalities. Later i came to know that the Guideline value of the residential site is appreciated/increased in the latest circular of Govt Of Karnataka to the value of Rs. 50 Lakhs before the date of agreement of sale. Now my question is does entering into sale deed below the GLR affects my ownership on the property, can i revise the Sale amount while entering into Sale Deed. Should i cancel the Agreement of Sale before entering into Sale deed or an additional clause can be added in the Sale Deed itself. As discussed with a CA it was told that purchase of a property below the Guideline value, purchaser has to apy full amount of applicable Stamp duty along with including the differential amount in the Income Tax statement under the Income from other sources by paying as a Capital Gain Tax. Kindly help me with ideal suggestions to resolve the issues.