The U.S. Office of Management and Budget is expected to decide soon whether an eight-acre parcel of vacant surplus federal land in Southwest Washington will be used for a warehouse or a Metrobus garage or sold by sealed bid.

The city was waiting to begin negotiations to buy the land from the General Services Administration for a Metrobus garage when the Smithsonian Institution, which as a federal agency has prior claim on surplus land, put in a request for it last summer.

Smithsonian officials want to use the property as a site to store building materials during construction of a new underground museum on the Mall, and later as the location of a new warehouse for exhibitions.

But the Smithsonian said it could not afford the estimated $5 million to $7 million market price for the tract, and asked GSA to give it the land free.

Free transfers of land among federal agencies were common until a year ago, when the Reagan administration changed the policy to require that surplus land be sold at market value.

Under the new policy, before a federal agency can obtain surplus land for less than market value, OMB must affirm that the transfer is in the best interest of the taxpayers.

Discussions about the property began in 1981, when Mayor Marion Barry asked GSA Administrator Gerald P. Carmen whether the federal government had unneeded land that the city could buy to replace its 44-year-old Metrobus garage at M and Half streets SW. At the time, Barry told Carmen the Half Street garage "is by far the worst in the Metrobus system" and "too small to operate efficiently."

Carmen's staff came up with the eight acres adjacent to Fort Lesley J. McNair at Second Street SW between Q and S streets, land once used as a parking lot for the Carter and Reagan inaugural committees, and began proceedings to sell it to the city.

At meetings last summer on the planned sale, the proposed bus garage drew cries of outrage from many Southwest citizens who said they feared the effects of 250 Metro buses plying their streets on the way to the garage.

Metro officials have since said the buses would be routed under the South Capitol Street Bridge through industrial sections that would avoid the residential area.

Another Southwest neighbor also opposed the idea--the Defense Department, whose college for advanced strategy is at Fort McNair. James N. Juliana, an assistant secretary of defense, wrote GSA that "the noise of several hundred buses starting up early in the morning and driving in and out of that area throughout the day would be intolerable to the residents and students at Fort McNair."

While GSA and District officials were trying to get through the complicated land disposal procedures, the Smithsonian heard about the Southwest property and asked for it--free of charge. GSA and the Smithsonian haggled for months and then, in late December, the matter went to OMB for a decision.

At about the same time, with city officials weary of the red tape and delay, D.C. transportation director Tom Downs wrote Smithsonian officials a letter saying that the city would not raise obstacles to the Smithsonian obtaining the land.

But Downs did not write to GSA, and GSA's assistant commissioner of real property, Earl C. Jones Jr., said all he heard "was that they wanted it."

Jones said if OMB rejects transfer of the land to the Smithsonian, "We'll go back to the city and give them another crack at it, even if they've told the Smithsonian they don't want it. If they say no again, 20 days later we'll begin sales procedures."

Meanwhile, Metro officials have been looking at other sites, but haven't lost interest in the land in question.

"I'm surprised to hear they're interested in talking now," said Albert J. Roohr, Metro's director of systems and service planning. "But I'm pleased. I hope we can make a deal."