The owners of the Washington Diplomats, who have hedged repeatedly in the last year when asked about the team's future in Washington, are negotiating to play an exhibition game in Milwaukee County Stadium next month.

Jerry Saperstein, chief aide to Madison Square Garden President Sonny Werblin, confirmed yesterday that Garden management has been talking to Milwaukee officials about an exhibition game, which would be played Sept. 29.

"We're just trying to keep the team active during the offseason," Saperstein said. "We're planning to play a number of games during the offseason. This would just be one of them."

At the same time yesterday, General Manager John Carbray announced that MSG had authorized a 30,000-piece mailing campaign, set to begin at season's end, for 1980 Diplomat season tickets at RFK Stadium.

"I can't make any promises about what Madison Square Garden is going to do but I would say this indicates pretty strongly where the Dips will be next season."

Weblin said earlier this season that the Dips will definitely remain in Washington next season even though The Garden has been disappointed with attendance (11,973 average for the regular season) at RFK Stadium. However, he has refused to make any commitment beyond that.

Madison Square Garden has been connected with Milwaukee since last summer when it was negotiating to purchase the Memphis Rogues. At that time, reports were that the Rogues would either move to New York or Milwaukee.

Werblin said earlier this summer that Milwaukee was "interesting. It's one of the major markets in the country and it doesn't have soccer."

Saperstein repeated that theme yesterday. "There are 1.5 million people in Milwaukee and no NASL soccer. We're also talking about playing exhibitions in other cities that don't have soccer." He mentioned Jacksonville, Indianapolis, Winnipeg and Montreal as cities the Dips might visit during the offseason.

Saperstein insisted that if the Dips play in Milwaukee in September -- they are scheduled to play in the Carribean most of the month -- it did not mean that MSG was testing Milwaukee as a possible home for the Diplomats in 1981.

But a number of club officials said yesterday that they saw the game as just that. "Whoever heard of a Carribean tour with a stop in Milwaukee?" said one.

And another added: "They're testing things, experimenting. They still don't really know what the hell they are doing up there."

And Coach Gordon Bradley noted: "I can't see any reason for playing in Milwaukee if we're going to be in Washington next year. But if they tell us to play there, we'll play there."

A final decision on the exhibition game, which would probably be against an NASL opponent, is expected in the next two weeks. Washington club officials are reportedly still trying to talk Garden management out of playing the game at all.