John Paul Davis, the author of the new book, cites scenes from A Gest of Robyn Hode, one of the earliest references to Robin Hood which dates from the 1500s, to support his theory.
In the ballad, Robin is approached by a knight who is indebted to an abbot and asked for a loan. Robin asks the knight if he has a guarantor, then agrees to give him the money, to be repaid over a year.
He asks Little John to count out £400 from his treasury.
Later in the ballad, which is written in Middle English, the knight returns to see Robin, and with his debts to the abbot cleared, offers to repay the loan together with an extra deposit charge.
Robin, however, declines the repayment, saying he has already received the money after stealing it from the abbot himself as a punishment for his greed, and tells the knight that it would be wrong to take the money twice.