The Fairfax County Board of Supervisors approved a new agreement yesterday with developers of the planned county government center at Fair Oaks, negotiating a lower rent that is still not low enough to please the board's chairman.

Board of Supervisors Chairman Audrey Moore, a Democrat expected to seek reelection this fall, cast the sole dissenting vote on the measure and accused its supporters -- including Thomas M. Davis III (R-Mason), who has announced he will oppose her -- of engineering a sweetheart deal for a developer to the detriment of tax-paying homeowners.

However, county officials defended the move, saying it will ultimately save the county money.

The board's wrangling took place against a backdrop of fiscal austerity as Acting County Executive Richard A. King outlined about $10.2 million in budget cuts needed to close this year's deficit, and as several dozen advocates of mental health programs sat silently with placards reading "Human Needs First."

The measure approved yesterday amended a four-year-old agreement between the county and the Smith-Artery Partnership, under which the partnership would build a new government center in exchange for county promises of $98.5 million in public land, cash and leases on two buildings owned by the partnership.

Under the original agreement, Smith-Artery is building the government center, which will be owned by the county, and has completed one office building for the county and a commercial office building nearby. Smith-Artery was supposed to build another office building for the county.

The company has asked the county to occupy both completed office buildings and allow it to delay construction of the third building -- which would save the company $12 million over five years, because leasing additional commercial space would be difficult in a weak market, county officials explained.

In exchange, the developer agreed to several concessions, including a rent reduction worth $2.3 million over five years and $762,000 in credit for property taxes the county will lose. In addition, the county will have the option to buy the two leased buildings for $125.6 million after five years.

But Moore argued that even with the rent reduction, the county would be paying too much for office space -- as much as $27.75 per square foot at a time when the going rate ranges from $15 to $19 per square foot.

Moore, who cast the only nay vote on the original 1987 deal between the county and the partnership, called it "a poor man's way of doing business with a loan shark." She slammed her Republican rivals who supported it four years ago -- Davis and then-Chairman John F. Herrity. She called the deal the "Herrity-Davis Monument to Bureaucracy."

"This is a rip-off," she said. "This is the worst damn deal I've ever seen in my life."

Davis countered that voting no would have cost the county the negotiated rent reduction. He also noted that all six of Moore's Democratic colleagues voted yes on the final motion. "She's just posturing politically," he said. "She has no constructive alternative. She's running against her own board."

County officials said the original leasing rates were accepted at a time when the real estate market was healthier. Those rates also included a reimbursement to Smith-Artery for building the main government complex.

"We have given you a win-win," Kenneth L. McVearry, senior vice president of Charles E. Smith Cos., told the supervisors yesterday. "You should take it and run."

At the same time, the cost-conscious board unanimously rejected a proposed contract to pay a consultant $258,000 to help the government move to its new complex.

General Services Director Fred K. Kramer, who said the outside help was needed, told the supervisors not to blame him if the move is difficult.

But the supervisors appeared to find it hard to ignore the silent protesters who spent the day holding up signs urging them not to cut mental health services.

Among the $10.2 million in budget reductions outlined by King yesterday was $1.3 million cut from the Fairfax-Falls Church Community Services Board, which will delay the opening of a group home and reduce the number of clients to be served in county mental health centers. Other cuts covered a wide range of items, from a new paper shredder to a delay in opening four child-care centers and a reduction in library personnel.