Although the future ownership of the Philadelphia Eagles remains in doubt, the National Football League team and baseball's Phillies have signed new leases that contain concessions from the city and guarantee the teams will play all home games in Veterans Stadium for the next 26 years.

An agreement on the lease concessions, which will cost the city about $15 million in stadium improvements, came in a package Mayor W. Wilson Goode put together in December to keep Eagles owner Leonard Tose from moving the club to Phoenix.

Yesterday, Philadelphia Flyers owner Ed Snider said a group of investors he has headed has decided not to purchase an interest in the Eagles, leaving the franchise on continued shaky financial ground.

Snider would not comment on the reason for his group's withdrawal but the Philadelphia Inquirer, quoting sources, said Provident National Bank, which was to provide much of the financing for the purchase, objected to Tose's retaining as much control of the team as he wanted . . .

Marcus Dupree of the Portland Breakers will miss the rest of the U.S. Football League season after surgery to repair ligament damage in his left knee.

Dupree, in the second year of a five-year, $6 million contract, tore a ligament in a 9-7 loss to Arizona Sunday in his team's season opener . . .

U.S. Football League attendance for the first weekend of the season surpassed last year's full-season league average but dropped by more than 3,000 per game from last year's opening-day turnouts.

Reported attendance at seven USFL games played Saturday, Sunday and Monday was an average of 29,233, compared with 32,918 for nine opening games last year and slightly more than 27,000 for the 324-game '84 season . . .

St. Louis County Executive Gene McNary and other county officials are studying several suburban sites for a proposed 70,000-seat domed stadium for the Cardinals.

Cardinals owner Bill Bidwill has said he will listen to offers from other cities interested in the Cardinals, but McNary said he thinks Bidwill will keep the team in St. Louis if he has a better facility than 51,000-seat Busch Stadium, the second-smallest in the NFL.