Iran had been expected to receive the first of 80 aircraft ordered from the U.S. planemaker in April 2018, but at least one brand-new aircraft is reported to be sitting unused because it is no longer needed by the Turkish carrier.
Industry sources said Boeing was in negotiations to release at least one 777-300ER originally built for Turkish Airlines, which is deferring deliveries due to weaker traffic following last year's failed coup attempt in Turkey.
Boeing said it never comments on talks with customers. The airlines involved were not immediately available for comment.
Iran's Deputy Roads and Urban Development Minister Asghar Fakhrieh-Kashan told the semi-official Mehr news agency the first Boeing 777 aircraft would reach Tehran within a month.
It would be the first new U.S.-built jet delivered to Iran since the 1979 Islamic revolution.
The long-haul 777 is worth $347 million at list prices but is likely to have been sold for less than half that, according to industry estimates.
IranAir has also ordered 100 aircraft from Europe's Airbus under a deal to lift most sanctions in return for curbs on Iran's nuclear program.
Its return to the aviation market after decades of sanctions comes at a time when airlines elsewhere are having second thoughts about purchases due to concerns about the economy and looming over-capacity among wide-body jets.
That trend has made a number of unused jets available for quick delivery at competitive prices, including three Airbus jets recently delivered to Iran, and has allowed IranAir to jump the usual waiting list of several years.
The government of pragmatist President Hassan Rouhani is seen as keen to showcase results from the sanctions deal ahead of a May election at which challengers include hardline Shi'ite cleric Ebrahim Raisi.
Aviation sources say the first aircraft were paid directly from Iranian funds, but doubts remain over credit financing needed to secure almost 180 jets still on order.
Western banks continue to shy away from financing deals between IranAir and Western companies, fearing U.S. banking sanctions that remain in force or a new chill in relations between Tehran and the West under U.S. President Donald Trump.
Boeing has stressed the benefits to U.S. jobs of the plane deals.
Fakhrieh-Kashan was meanwhile quoted on Monday as saying IranAir had reached a long-awaited agreement to buy 20 European turboprops from ATR.
Talks over maintenance with engine maker Pratt & Whitney Canada had delayed a final deal. It was not immediately clear whether the official was referring to an earlier deal for the planes or the final contract including engine overhaul.
ATR said on Sunday it was still in talks with IranAir.
(Tim Hepher and Dubai Newsroom - Reuters)