Land Sale agreement Contract

Sir/Madam , My father was battling Cancer in the year from 2016 . He lived seperately on house quarrels. He bought a plot of 10K ( Old) of land which is just 100m away from the house I live with my mother. In 2017 , he needed money for cancer treatment, so he decided to sell a portion of land to a buyer . The land sale agreement of only 12.5 L ( Old) was identified by a lawyer and both my father and the buyer signed it in Nov /2017 and my father received a initial consideration of Rs. 3 Lakhs out of 8.5 lakhs. In March/2018 , my father died .The land sale agreement got sub- registered in the year Dec/2020 by the buyer after my father's death. My mother and Me were not the witnesses of the said land sale agreement and we did not know of the same untill 2024/Dec. As I and my mother lived seperately from my father , my uncle took that opportunity and took control of that land but somehow later we entered into a mutual understanding and the said property got registered in my mother's name as the legal heir in the year 2023/Nov. Now the buyer is forcing us to execute the sale deed in his favour . He even threatens to go to court against my mother . Now .............. A) The buyer did not contact me and my mother during the land sale agreement in 2017 .He only contacted us after almost 8.5 years. B) There's no mention in the Land sale agreement contract about the legal heirs or representatives who will be responsible to execute the agreement after the seller's death. C) The time limit is not set between the land sale agreement and the sale deed execution . D) There is no mention in the land sale agreement of length and breadth ( dimensions) of land the buyer wants to buy but only the boundary is mentioned. E) The land sale agreement was signed in the year 2017 , my father died in 2018 and the land sale agreement was sub - registered in the year 2020 .....How is this possible?????? As legal heirs ....what should we do ????? I hope you will guide me regarding this....