PORTLAND — After nearly three months of negotiation and more than a year of public debate, the city has reached a tentative agreement to sell two-thirds of Congress Square Plaza to the owner of the former Eastland Park Hotel.

RockBridge Capital LLC would pay more than $523,600 for about 9,500 square feet of the public space at the corner of Congress and High streets, according to a memo by Greg Mitchell, the city’s director of economic development, that was released Aug. 16.

The purchase price is based on a market appraisal of $55.12 per square foot.

If the sale is completed, about 4,800 square feet of the plaza would remain as public space.

RockBridge and co-developer New Castle Hotels & Resorts are finishing a $50 million renovation of the 86-year-old hotel, and plan to reopen it in December as the Westin Portland Harborview Hotel. The developers have hoped to acquire a portion of the plaza to build an adjoining center for meetings and events.

Terms of the draft agreement include:

• The meeting and event center must be in “substantial conformance” with a design that was presented to the City Council’s Housing and Community Development Committee on May 29. At that meeting, the committee voted 3-1 to negotiate sale of the open space.

• Once the center is built, RockBridge must continue to use it for meetings and events for 10 years. Afterward, it could be converted to “uses customarily found in hotels.”

• A public clock housed in the plaza, once part of the now-demolished Union Railroad Station on St. John Street, would not be included in the sale.

• RockBridge will contribute $45,000 to the city to pay for infrastructure improvements in the plaza, including a new sidewalk along Congress Street.

The HCD Committee is scheduled to consider the draft agreement at a meeting on Wednesday and may then make a recommendation to the City Council, which has the final decision-making authority on the sale of city property.

The Friends of Congress Square Park, a neighborhood group that has been leading opposition to the sale, has requested time at the meeting to deliver a presentation, according to a memo from the group’s president, Frank Turek.

The Friends have also been surveying the public to gather ideas for improving the plaza without selling the space, and plan to hold a charette on alternative designs for it on Aug. 31. Meanwhile, the city has been surveying the public, too, gathering suggestions for overhauling the entire area of Congress Square, including the plaza, the intersection of Congress and High streets, and public space near the Portland Museum of Art.

The council could take up the agreement at its next meeting on Sept. 9, and if the agreement is approved, the sale could be completed by early October, Mitchell said Monday.

The committee meeting is scheduled for 5:30 p.m. in council chambers at City Hall.

William Hall can be reached at 781-3661 ext. 106 or whall@theforecaster.net. Follow him on Twitter: @hallwilliam4.