Agricultural lease deed

My father had a agricultural land sale agreement with brother for 6 lakhs in 1994. Family issues had held him from filing a suit within the stipulated period. Now,after retirement, he asked his brother for registration but he declined. He thus sent an advocate notice DT 5th October 2017 to brother asking for registration or else he would have to file a non performance suit with the court. His brother has today 25.10.17 sent his reply saying that all alligations are false and the sale deed was a trick. We should not be stepping on this land or pick any contractual money or he will suit us. How legitimate is this answer and do we loose our rights? Do note that the sale deed was on a 100 rupee stamp with notary and witness signatures. My father himself is contractually handing the land to tillers annually and has all proofs on stamp and in black and white.