The two companies signed the deal at a ceremony on the sidelines of the biennial commercial and defence-focused Singapore Airshow.
The order came after Tibet Airlines and COMAC in December announced that they would jointly research a variant of the C919 suitable for high-altitude plateaus.
Henan Civil Aviation Development and Investment Group on Tuesday also signed an agreement with COMAC for six ARJ21 derived models that include firefighting, medical services and emergency management.
COMAC's C919 is on its first trip outside Chinese territory in Singapore, and the Chinese planemaker is positioning the jet as a viable alternative to the Airbus A320neo and Boeing's 737 MAX single-aisle jet families, as they struggle to meet demand for new planes, and Boeing grapples with a string of crises.
The plane is only certified within China and the first of now four C919s began flying with China Eastern Airlines last year.
COMAC has received more than 1,000 C919 orders so far, state media reported in September, mostly from Chinese airlines and lessors.
In September, GallopAir, a new Brunei-based airline, said it planned to buy 30 aircraft from COMAC, including the C919, which would mark the narrow-body jet's first overseas purchase.
Little is known about GallopAir, which is expected to be only the second Brunei-based airline operator after national carrier Royal Brunei Airlines. The company has said it intends to provide flights in the Brunei Darussalam-Indonesia-Malaysia-Philippines East ASEAN growth area.
Once completed, the deal would make GallopAir the second international operator of Chinese-made aircraft after Indonesian low-cost carrier TransNusa, which operates a small fleet of the ARJ21 regional jets.
COMAC is also showcasing a shortened C919 variant fit for high-altitude plateaus and a stretched variant at its airshow booth, and has also brought the ARJ21 for a flying display.
(Reporting by Brenda Goh; Editing by Muralikumar Anantharaman and Gerry Doyle)
