Senior congressional tax negotiators are demanding that the federal government seize 540 acres of the northern Virginia countryside to block construction of a shopping mall on the spot where Gen. Robert E. Lee led one of the bloodiest battles of the Civil War.

The negotiators, who are seeking compromise on competing House and Senate versions of a catchall tax bill, agreed Thursday night to include a requirement that the land be annexed into Manassas National Battlefield Park, 30 miles west of Washington.Leaders of the Senate-House conference committee, hoping to wrap up work on the bill Friday, also agreed Thursday to provisions:

-Imposing limited economic sanctions against Iraq in protest of that nation's use of poison gas against thousands of members of its Kurdish minority.

The sanctions would prohibit export of munitions and high-technology equipment to Iraq and require U.S. representatives to oppose lending to Iraq by the World Bank and other international organizations. If the president has evidence at year-end that the use of chemical weapons is continuing, he would have to impose tougher sanctions that could include a cutoff of import-export credit and barring Iraqi imports into the United States.

-Giving freelance writers, photographers and artists relief from a part of the 1986 tax-overhaul law. Under the new provision, they could deduct certain expenses of producing a book, painting or photographs before the works started generating income.

-Increasing, to 33.75 cents per mile, the mileage allowance given rural mail carriers who drive their own cars on their routes. Most people who use their car in business get only 22.5 cents a mile.

-Removing restrictions on the amount of long-term bonds the Treasury Department may issue at interest rates above 4.25 percent.

Agreements on the provisions were reached by Rep. Dan Rostenkowski, D-Ill., chairman of the House Ways and Means Committee; Sen. Max Baucus, D-Mont., chairman of the Senate Finance revenue subcommittee; and the senior Republicans on the two committees, Rep. Bill Archer, R-Texas, and Sen. Bob Packwood, R-Ore.

The agreements are subject to approval by a majority of the other negotiators, but aides said they expect no problem.

The chief purpose of the differing bills is to fix hundreds of errors and ambiguities in the new tax law enacted in 1986. The measures have been expanded to carry scores of relatively minor tax benefits and the tax increases necessary to pay for them. The Senate version also includes a "bill of rights" to help taxpayers who face enforcement or collections actions by the Internal Revenue Service.

"We made a lot of progress. We are doing very well," Baucus said at the end of Thursday's negotiations.

Added Rostenkowski: "We've got $7 billion" worth of tax benefits in the House bill and "the Senate's got $3 billion. It (the final version) will be some place in the middle."

The provision requiring the taking of a portion of the site where the Second Battle of Manassas was fought in August 1862 was added to the tax bill only because sponsors saw no other way of getting it enacted into law this year.