In these days of fiscal restraint in government, Fairfax County is about to come into some easy money. About $500,000, to be precise.

The county recently bought 33 new buses for about $4.1 million to serve commuters using the Huntington Metrorail station. Now, under a provision in federal tax law, Fairfax is selling the tax benefits of those buses to the highest bidder.

No property changes hands, and the maneuver is a paper transaction only. "All I know is that we'll probably get about $500,000 out of this," said county transportation chief Shiva K. Pant. "Quite a chunk of change."

Local governments and transit authorities have been selling the tax benefits of their newly aquired buses and trains since 1981, when Congress authorized the practice, which is known as "safe-harbor leasing."

Companies buy the depreciation rights and use them to shelter income and reduce their tax bills. Because local governments and transit authorities are not taxed, they lose nothing by selling the tax benefits.

"It's a very intelligent tool for counties to use, because it's free money," said Michael W. Harling, a broker in Dallas who was hired by Fairfax and Montgomery counties and Metro to find buyers for the tax breaks. "Sometimes it's hard to convince people that it's so easy."

The apparent loser is the federal Treasury. In the current fiscal year, the government estimates it will forfeit nearly $2 billion in taxes from safe-harbor leasing deals. Much of that sum will come from the sale of depreciation for mass transit vehicles.

In this region, Metro has used the provision extensively, selling off about $5.1 million in tax benefits since September 1983, according to its treasurer, Alvin W. Doehring.

"We sell to anybody," said Doehring. Metro has sold tax benefits to banks, a potato company in Idaho and liquor distributors, he said, whoever is the highest bidder.

Alexandria recently sold the depreciation on 17 new buses for $267,000, according to Deputy City Manager Clifford H. Rush. Montgomery County made $624,000 when it sold the depreciation rights of 41 Ride-On buses that went into operation earlier this year, county Finance Director Max Bohnstedt said.

Local governments and transit authorities typically sell the tax benefits for 10 to 15 percent of their investment. The income from the sales is usually returned to the operating fund.

Last week, Harling placed a small classified advertisement in The Wall Street Journal for the tax benefits on Fairfax County's 33 buses.

The ad carried county transportation chief Pant's name. In the space of 48 hours, Pant said, he had received about 15 inquiries from the ad. Harling said he expects the depreciation rights will be sold by the end of September, when the buses go into operation.

"We're not dealing with a loophole or anything like that here," said Harling. "We're dealing with a specific regulation intended" to help mass transit and the private sector.

It all may be too good to last, however. The law authorizing the procedure expires Jan. 1, 1988. Deficit-cutters on Capitol Hill with an eye to boosting revenue may not want to renew it.