Nama binned a proposal from Peter Robinson suggesting Northern Ireland debtors be released from personal guarantees on their loans.
The First Minister submitted a "letter of intent" - dubbed a debtors' charter - in January 2014.
It was signed by Jeremy Gardner, a private secretary to the First Minister.
Sinn Fein said it was sent without the knowledge of Deputy First Minister Martin McGuinness.
The letter appeared to summarise an agreement between Pimco, one of the early bidders for the Nama portfolio, and the Executive.
Its purpose was to require the purchaser of the portfolio to enter into a memorandum of understanding (MoU) with the Executive. Nama chairman Frank Daly said it sought confirmation of "certain fundamental conditions relating to the future management of the portfolio".
The letter referred to personal guarantees, adding they were an "issue of acute political sensitivity".
He said Nama did not engage further in relation to the letter.
Mr Daly told TDs he had been puzzled by it. "This MoU has no status within Nama whatsoever," he said.
Mr Daly added: "We would never, ever have countenanced this as a condition of sale. It would be a debtors' charter and we would never have countenanced it."
Correspondence from the Ofiice of the First and Deputy First Minister (OFMDFM) requires the approval of both Mr Robinson and Mr McGuinness.
A Sinn Fein spokesperson said: "The correspondence referred to did not have the approval, consent or knowledge of the Deputy First Minister and therefore was not a formal communication from the Office of the First and Deputy First Minister."
The DUP said it was a matter for OFMDFM, which did not acknowledge or respond to inquiries from this newspaper.