Fradulent sale

My friends have 8 acres land near Hyderabad and 3 brothers and 1 sister, muslims. 1998 all three brothers after the death of their fathers, want to sell their land, got deal for Rs. 68000 for all 8 acres. The purchaser only gave Rs.20,000 in cash, got all the 3 brothers signed on blank Rs.100 stamp paper. Later he didn't give any money. The purchaser started using the land. As 3 brothers are illiterate and have no influence in the village,they don't object. The purchaser prepared a sale deed on stamp paper for total 8 acres, got mutation for 6 acres, as the brothers sister didn't sign the stamp paper. Now the children from 3 brothers are majors and it's a ancestral property. is there any way to claim back property by 3 brothers and their children. No sale deed was registered in registrar's office.For your replies thanks in advance