Japanese firm Mitsubishi Heavy Industries has agreed to buy Bombardier's CRJ aircraft programme for $550m (£432m).
In May, Bombardier put its entire Northern Ireland operation up for sale.
Bombardier employs about 3,600 people across several locations in Northern Ireland.
It said it was reviewing the impact of the sale on the Northern Ireland sites and would evaluate ways to mitigate any potential effect on the workforce.
The central fuselages of the CRJ planes are made at Bombardier's Belfast plant.
It's understood about 300 people work directly or indirectly on production of the central fuselage for the CRJ jets, the majority of which are based at Queen's Island.
But the trade union Unite said it was concerned about what the sale might mean for long-term future of the staff in Belfast.
"At worst, this sale might simply be a case of Mitsubishi buying up a competitor to increase market share," said Unite's Susan Fitzgerald.
"Unite are calling on Bombardier to guarantee this sale will be tied to a cast-iron commitment to the livelihoods of those employed in CRJ production.
"The UK government must be proactive in defence of these vital jobs and skills."