Sc/st land purchase in 1990

Dear Experts,

I would like yo ask a query, that my father purchased a land in Utter Pradesh (Kanpur) in 1994 and the nutation (दाखिल खरिज) and other formalities were done in next 1 years.

Now the seller is not  alive. 

Now someone from third party is complaining that the land belongs to SC/ST. (it was not mentioned in sales deed and nutation that the land belongs to SC/ST)

I request you to please advice me, whether this is valid purchase or not. 

Thanks for your valuable advice,

Regards, Ayush Awasthi