Cancellation of registered deed

My father sold a property to a builder and within two months he died in suspicious condition.till then he was critically ill. the registry was also done at private residence in absence of any family members ,friends or known persons.we didnt had any copy of the agreement.verbally we knew half amount was paid and rest half was to be paid.the builder was assuring that he will pay within few days. but in the mean time my father expired. later when we approached the builder he said full amount has been paid.after obtaining the certified copy we were shocked to see that the half amount he paid was the total consideration amount in the deed. later on reading the deed it was found that the deed was undervalued .the government valuation was half the price of the actual value.after that written complaint was given to the registrar but no action was taken for more then six months ,after that written complaint was given to district collector regarding the same. after that the revaluation was done and a notice was served to the builder for deficit of stamp duty. goverment will claim there deficit .how will we get our compensation which was due to the negligence of the registrar.