Delaware North is selling its fully-permitted, 40-story residential project in the Bulfinch Triangle to luxury apartment developer AvalonBay, according to sources.

The firms have a purchase and sale agreement and are in the due diligence period. However, assuming the deal goes through, construction on the 550,000-square-foot project could start almost immediately.

The project, which was permitted in 2005, was stalled in 2007 as the recession set in. But Delaware North did all the heavy lifting to see the project through its city and state approvals and is hoping to now hand it over to AvalonBay.

"It was understood from the beginning that … they never promised that they would develop it themselves," said one source that wished to remain anonymous. "They’re not residential developers."