The Port Authority’s board is expected to approve an agreement with global shopping center owner Westfield Group for a 50 percent stake in the World Trade Center’s sprawling retail space that’s not expected to open until 2015.

Last July, the company and the agency announced an agreement in principle for 365,000 square feet, with an additional 90,000 square feet to be included once Tower 2 is completed.

The venture is valued at $1.225 billion with Westfield funding $612.5 million in increments as progress is made with development and leasing. Westfield has also agreed to provide an additional $37.5 million based on “the achievement of agreed yield targets.”