Increase in area of sold property by our ancestors without our concent

My grandfather sold one adjecent house in 1926 to our relative in Pune city. After few days purchaser written letter to mamledar(govt) and asked to increase area of the house. There is no signature obtained by purchaser from seller(My grandfather) this letter. Mamledar agreed to this and changed the area. The old copy of sale deed we have found recently and then we came to know this fact. Is this legal to change the area without signature of seller. Now what we can do legally to decrease the area. What can be done?