Purchasers of our land has still not paid the money what to do

Hello Sir/Mam, Actually land owner is my grand mother,overall she had 8.34 acres land. but My father sold his share(2.08 acres) to remaining 3 brothers. Is that registration valid..?, because that land was not on my father name, even he had sold it. Even in sale deed also my father is vendor and remaining three are purchasers. Why I am writing this question is who are bought my property which was on my grand mother name they had not given amount till today. We had sold it 5 years back, on the day of registration we have a agreement on 10rs bond paper to pay the land amount in 45 days, but they did not pay to us. Please help me to proceed further on this.. Can I legally proceed for my share in my grand mother land..? Please suggest which is best way to proceed.