Challangeing regisrered sale deed

two person 'a'(buyer) and 'b'(seller) did registered conditional sale deed of a agriculture land in 2006. according to conditional sale deed if 'b' could't pay 100000( total market value in 80000 in govt record) rupee till 25 october 2010 then conditional sale deed convert into real sale deed and buyer 'a' will we the absolute owner of property and get possession and mutation in government record. seller 'b' didnot pay 100000 till before and after the specified date in the deed. so as an owner of land buyer 'a' sold land to 'c' (who is my father) in 10 march 2012 by registered sale deed. new buyer 'c' get mutation(dakhil kharij) of this land on his name and get possession of land in april 2012.since april 2012 to 2019 'c' paying property tax continusally and doing cropping and cultivating of land and possesed. also 'c' get agriculture loan on this land.now the problem is that the son of previous seller 'b' give a notice through his advocate to 'a' and 'c'that buyer 'a' have no right to sell the land to c(my father) because it is a conditional sale deed. can my father 'c' face any problem in future. we have registered sale deed,mutation in government record,property tax,possession etc please reply sir....