The Metropolitan Washington Airports Authority board on Wednesday postponed a vote on removing a union-friendly incentive for contractors bidding to build the second phase of Metro’s new Silver Line.

The board said it plans to call a special session no later than June 6 to discuss the 10 percent bonus it had made part of a project labor agreement for the planned construction of the rail line’s extension into Loudoun County.

The board’s vote could prove to be pivotal. Virginia Gov. Robert F. McDonnell (R) has said that if the incentive remains, he will withhold $150 million that the state has promised to help pay for the construction of the second phase of the Silver Line.

Supervisors in Loudoun County, who have pledged more than $200 million to the project, have said they are reluctant to fulfill their county’s commitment if the incentive remains. Loudoun has until July 4 to decide whether it is in or out of the project.

But MWAA’s board has not shown any inclination to drop the incentive. U.S. Transportation Secretary Ray LaHood had stepped in to try to bring the parties to an agreement but did not reach a final deal.

“Virginia is not holding up the $150 million,” said former Congressman Tom Davis (R), an MWAA board member, after the board meeting. “We are. We’re trying to work this out. My concern is we want Loudoun in and we want the state’s money.”

“If we don’t,” change the labor agreement, Davis added, “we would see the $150 million evaporate and Loudoun will not participate.”

With 1,400 employees and a nearly $2 billion annual budget, MWAA oversees Reagan National and Dulles International airports and the construction of the Silver Line. The first phase of the new Metro line is under construction and is expected to be completed in August 2013.

On Wednesday, MWAA’s joint finance and Dulles corridor committee also approved spending another $150 million on the first phase of the project because it has hit cost overruns, mostly because of Metro’s added safety requirements.

MWAA’s board chairman Michael Curto said he expects the discussion on the project labor agreement to be a “full, frank, open discussion.”