Property sale by private limited company

My mother had bought a plot of land from a real estate company registered as a private limited company in 1985. My mother is selling the land now and has found a party. The prospective buyer's lawyer has raised the following query. 1.The land was bought from MRS X through a power of attorney deed represented by 3 directors A,B and C of the company in 1978. 2. The company subsequently planned a layout, got necesary approvals and has sold one of the plot in the layout to my mother in 1984 signed by the director D who is A's son. The party is now asking why their is only 1 director who has signed the sale deed. The people who has bought the land and who have sold it is different. I tried to explain that the people have acted on behalf of the company and not in their individual capacity. The company owned the land and eventually the company had sold it. The party is asking for a letter from the company to clarify. In such a situation. I need advise as to what as to be done and is their any necessity to produce any letter and if yes, what should the letter speak about. ( The company is a reputed one and so many transactions has happened in the same layout in the between years. It is more a technical issue than the matter of genuineness)