Rectification deed alternatives

About 30 years back my father was allotted a MIG house site (267sq.yds + extra land as corner plot in the layout) through his company’s house building society. He retired 12 years back from his services. After 10 years of initial registration, had asked the society to register the extra land as he wanted to construct 1st Floor. Then the society’s secretary had given a letter signed on the society’s letterhead that there was an error mentioning the extent of area in the sale deed but the total extent of land, as per the schedule mentioned in the sale deed, belongs to my father only. Based on this letter the municipal authorities have sanctioned the house plan and the house taxes are paid regularly from that time onwards. Now , when we wanted to sell the house site as a whole, the buyer is insisting the extra land to be registered for obtain bank loans etc., the document writer also says it needs a rectification deed, signed by the society, for getting the extra piece of land to have a pukka document. The question is the society has closed its project files subsequently and the present secretary has no idea of that project (being a project of 30 years old) and denying entering for rectification of the deed. Further he says consider the society is dead. Under these circumstances what are the alternatives? Can the registration authorities or any authorities pass any orders for rectification so as to obtain a legal ownership of the entire site?