The budget deal hammered out yesterday by congressional negotiators and the administration has saved federal employees from furloughs -- at least for the time being -- but part of the plan would eliminate the popular lump-sum federal pension option.

Employee unions and civil service advocates in Congress said that if the provision is approved, they would expect an exodus of retirement-age workers from the government in October, before the option is eliminated. But both groups also said federal employees come out better in the budget proposal than had been expected when the prospect of freezing cost-of-living adjustments loomed last week.

Under the popular lump-sum option, retirees receive two payments -- one shortly after they retire and the second a year later -- equal to the amount they have contributed to the pension plan. Although between 85 and 95 percent of the payment is taxable, nearly 70 percent of all retirees have chosen to take the lump-sum payments. In the Washington area, the average payment is about $32,000.

"There is no doubt it's disappointing" to lose the option, said John N. Sturdivant, president of the American Federation of Government Employees. "But if that's the only hit we've taken, I think we've been pretty successful."

Under the proposal, which now needs to be adopted by Congress and signed by the president, employees have until Nov. 1 to retire and still retain the lump-sum pension option. The 30-day window is considered a compromise to mollify lawmakers with large numbers of federal employees in their districts.

"I think {federal employees} are not going to be terribly unhappy with the result," Rep. Constance A. Morella (R-Md.) said yesterday.

"I am pleased we have been able to reduce the impact on federal employees and retirees," said Rep. Steny H. Hoyer (D-Md.).

The Office of Management and Budget and the Congressional Budget Office calculate that eliminating the lump-sum option will save the government $1 billion in 1991 and $8.1 billion over five years.

Josh Neiman, legislative director of the National Federation of Federal Employees, yesterday questioned that figure. He predicted that so many employees may opt to retire before the Nov. 1 deadline "that it is going to cost them

more" than leaving the option unchanged.

The House and Senate convened in rare Sunday sessions to consider the budget proposal, the result of 15 weeks of negotiations between congressional leaders and the administration. Both the House and Senate last night passed a continuing resolution to fund the government until Oct. 5.

The measures avert a shutdown of the government for lack of funds and postpone until Oct. 5 furloughs that would take effect if lawmakers and the administration fail to agree on a deficit-reducing budget as outlined under the Gramm-Rudman-Hollings law. About 1.1 million federal employees received furlough notices last month.

The Oct. 5 deadline gives members of Congress time to study and debate the budget proposal. But by Friday they must either adopt a budget or pass another stopgap funding measure to avert a shutdown and furloughs.

Labor leaders yesterday lambasted the precarious position of federal employees in the budget-making process and called for the repeal of the Gramm-Rudman-Hollings law.

The budget deal "removes the threat of furloughs but it doesn't address the damage leading up to it, the anxiety, the pain," said Robert M. Tobias, president of the National Treasury Employees Union. "I woke up this morning furious over the cynical way the president has treated federal workers."

Yesterday a handful of NTEU employees protested outside the Capitol.

The budget proposal includes other changes related to employee health care and retirement:

Medical payments for the 1 percent of the over-65 age group currently not covered by Medicare -- civil servants who retired before 1984 under the Federal Employee Health Benefits Program -- will now be capped at the Medicare limits. The expected saving would be about $1.1 billion, according to the OMB.

The Postal Service will have to pick up the employer health care premiums of former employees who retired between 1971 and 1986, a savings to the government of about $4.2 billion over five years.

Beginning in 1994, the Postal Service would also be required to increase its contribution to the Civil Service Retirement Fund to pay for COLA benefits that some current employees will receive after they retire. The savings are estimated at $1.18 billion over five years.

The new charges to the Postal Service would effectively wipe out the $600 million postal officials figured the service earned this year through increased productivity.

"It does seem that the Post Office is being hit very hard," said a spokesman for the House Post Office and Civil Service Committee.