Is simply asking for a mod acceptable? Or, must the contractor obtain evidence of agreement on the part of the buyer (PCO)?
The following input was provided based on further discussion with the question submitter:
Under a cost reimbursement contract containing the clause at FAR 52.232-20, a change order would be issued (assuming the Government adds money to the contract), which would be a unilateral modification. I'm not aware there is anything to actually "negotiate." There may be some exchanges between the contracting officer and contractor to ensure the increased estimate cost is reasonable, but that is not necessarily a negotiation. FAR 52.232-20(b) requires the contractor to "...notify the Contracting Officer in writing..." and (c) to "...provide the Contracting Officer a revised estimate of the total cost of performing this contract." That is what the clause requires in terms of "asking for a mod."