My grand father purchased a property in 1932 and part of that given to his younger brother

My grand father purchased a property of 10 decimal in 1942 and a small part of that, 3 decimal given to his younger brother without any documents. My grand father and his younger brother are no more . My grand father's younger brother managed 5 decimal which is more than what he owned which got entered in register 2 in his name. This is learnt recently. In 1999 my grand father's younger brother's son sold the whole 5 decimal to another person however this is physically only 3 decimal. In his sale deed it is written that this property is खरीदगी का खास हिस्सा है. I came to know recently this thing . The purchaser of this property did mutation in 2016 of 5 decimal of the property. he claims that since he has purchased 5 decimal, He want to forcibly capture remaining 2 decimal from my property which is owned by me . (none municipal area) My question is- Can I go for deed cancellation and how to prevent the buyer who is claiming my property of his part?