Seller Has GPA But Sold On His Capacity Not Mentioned GPA In Sale Deed

One Person has GPA from all his sisters for making sale deed. But the Seller without incorporating GPA, sold one property sold on his capacity after the GPA date. Now for a safety, I want to make one affidavit/declaration as a notarized deed to keep as a record where Seller has by mistake not mentioned about GPA and the same deed to be considered as if made on GPA capacity and in case of any claim in future, Seller will be responsible for all losses. So which type of deed will be appropriate in such a case.