Fairfax County planner and a private developer have settled a major transportation dispute, enhancing prospects that the long-delayed Tysons II shopping mall will be built at the county's sprawling - and badly congested - Tysons Corner.

If the new mall is built, the area that some county officials say is becoming Fairfax's "downtown," would become the largest retail shopping complex in the Washington suburbs and one of the biggest shopping malls in the world. It would contain 2 million square feet of enclosed space.

County officials have delayed approval of developer Theodore N. Lerner'r palns for the second mall for years, citing objections to the massive amounts of traffic that the new mall would place on the already crowded roads in the area.

Yesterday county planners said the county's major objection to the new mall has been removed with Lerner's agreement to donate 8 acres of land for a freeway-style interchange at Dolley Madison Boulevard (Rte. 123) and Internation Drive, which leads to the 107-acre parcel where Tysons II would be built.

The agreement will leave the final county decision on the mall up to the Fairfax Board of Supervisors. County officials said the new agreement makes it unlikely that the county now can reject Lerner's rezoning application for the site.

If the board, which is scheduled to consider the rezoning on June 26, reject Lerner's rezoning application for the site.

If the board, whkich is scheduled to consider the rezoning on June 26, rejects Lerner's proposal, he will probably take his case to the state courts, claiming, as have other developers, that he is entitled under Virginia law to "the highest and best use" of his land.

County planner Richard Faubion also said the cost of the interchange has been reduced to $3 million through design changes. He said one of the few remaining questions is how much Lerner will contribute to the cost of construction.

John F. Herrity, chairman of the Fairfax supervisors, said yesterday,"I'm still skeptical," despite the general agreement reached by county planners and Lerner.

According to an analysis by county planners, "the potential for regional retail facilities at Tysons Corner is expected to expand well beyond the capacity of the existing mall, and possibly even beyond the capacity of (Tyson II)" in the coming years.

Although no prospective stores have been announced for the proposed Tysons II, Lerner's attoney, John T. Hazel Jr., has said, "Tenants are lined up clamoring for space. There will be a minimum of four department stores, and maybe five."

While some county supervisors are not enthusiastic about creating a giant shopping complex at Tysons Corner, their most serious concern has been transportation. Nearly 50,000 cars have to squeeze their way through the Tysons intersection every day, causing massive traffic jams.

A second mall would generate more than 30,000additional cars the county planners say. But the additional traffic load would be almost as heavy, according to the planners, if apartments and office buildings were constructed there instead of a mall. Under the present zoning, apartments and office buildings could be built on the site.

Relief, though, may be on the way. Congress is considering legislation that would permit the construction of parallel lanes along the nearly Dulles Airport Access Road, which is now open only to airport traffic. The parallel lanes, which would be built by Virginia and probably require a toll, would remove through-traffic from the crowded roads around the present shopping complex and the site for Tysons II.

Tysons Corner has become Fairfax's urban center - its downtown for Fair-fax County and maybe for Northern Virginia, according to Supervisor Martha V. Pennino (D-Centreville). Besides the present shopping center, it includes the Westpark office complex, a number of other office buildings and the five-tower 1,240 unit Rotonda condominium apartments.