Mayor Marion Barry offered several months ago to extend the Washington Redskins' lease for RFK Stadium if Redskins owner Jack Kent Cooke would commit to building a football stadium for the team in the District, sources familiar with the negotiations said yesterday.

Barry's offer would enable the Redskins to remain at RFK until they move into the new facility, the sources said. Their lease expires at the end of this season, and Cooke has offered to build a stadium that sources have said would be located on what is now RFK Stadium's Parking Lot 6.

The D.C. Armory Board and the Redskins are continuing to negotiate the terms of the deal for the new stadium. The Armory Board is scheduled to meet Thursday. Barry chairs the board, but likely will offer his resignation when his term as mayor expires next month and Sharon Pratt Dixon takes office.

Meanwhile, the D.C. Council today is expected to approve a long-term lease between the Armory Board and Metropolitan Washington Baseball, the group of investors seeking a baseball expansion franchise for RFK Stadium. That lease could make it difficult for the Redskins to use the stadium beyond the 1993 season.

Under the baseball lease, which would go into effect if Metropolitan Washington Baseball is awarded one of the two teams the National League will add in 1993, there could be renovations before the 1994 baseball season that would reduce seating capacity by 2,000 to 2,500, a source said. The removal of seats would allow the construction of skyboxes.

A reduction in seats would be a problem for the Redskins, who sell out every game and have a considerable season-ticket waiting list.

However, a source said the Armory Board would do whatever is necessary to accommodate the Redskins, and Metropolitan Washington Baseball organizer John Akridge has said he would be willing to work out conflicts with the Redskins.