The Michigan Panthers of the new U.S. Football League yesterday offered running back John Riggins a three-year guaranteed contract worth $2.5 million, according to two sources close to the negotiations. The offer is about 2 1/2 times more than Riggins' 1982 salary of $330,000 with the Washington Redskins.

It also was learned that Redskins starting linebacker Rich Milot, also a free agent, has been offered a contract by the USFL's Philadelphia Stars. Contract figures were not available. The Redskins have yet to make a counteroffer, according to Richard Bennett, Milot's lawyer.

Riggins, 33, most valuable player in the Redskins' Super Bowl victory last month, told Shire Rothbart, the team's vice president, and Jim Spavital, its general manager, he would talk to the Redskins before deciding. Spavital said yesterday's offer dealt with generalities and "a lot of other things need to be negotiated."

Riggins is supposed to come to Redskin Park Monday to talk to General Manager Bobby Beathard, who said he expects negotiations to begin Tuesday.

Riggins was unavailable for comment last night and Dick Berthelsen, NFLPA legal counsel who said he is assisting Riggins in negotiations, declined to discuss substantive issues from the meeting held at the Panthers' offices in Bloomfield Hills, Mich. Berthelsen described the meeting as "positive and interesting."

Berthelsen: "It's fair to say they gave a strong expression of interest in John playing in the USFL and regard signing him as one of the things that would give them instant credibility."

On Wednesday, the USFL's New Jersey Generals signed Heisman Trophy winner Herschel Walker to a three-year contract worth about $5 million. He had one year of eligibility remaining at the University of Georgia.

Riggins is representing himself in the latest negotiations, as he has done previously, Berthelsen said. "It's typical to be assisting players in the negotiations, something I do all the time. There's nothing better than helping a player improve his working conditions," he said.

USFL sources have said recently the Panthers were using the Riggins offer to sell tickets and Riggins was using it to improve his bargaining position with the Redskins.

Asked about this yesterday, Spavital said, "We need a big running back, and that's what he is."

"The USFL is for real and actively seeking NFL players," Berthelsen said. "These are not a lot of rumors being bandied about."

Redskins officials have said they planned to sign Riggins, who sat out the 1980 season in a salary dispute. Beathard said last night, "We plan to sign him, but I really don't know. We lost (Larry) Csonka (to the World Football League, when Beathard was personnel director of the Dolphins) when we didn't want to lose him."

Beathard said he would not discuss the reported salary offer made to Riggins until he was sure of the numbers.

Team owner Jack Kent Cooke declined comment.

Spavital declined to discuss exact figures in the Panthers' offer, except to say that three years for $2.5 million was high. Spavital coached Riggins to his first 1,000-yard season with the New York Jets.

"I think it went real good," Spavital said of yesterday's meeting. "We felt like he was interested in the USFL and the Michigan Panthers, in particular. We made him an offer and he was receptive."

Asked if Riggins had said he felt an obligation to play in Washington next season, Spavital replied: "He is interested in the Michigan Panthers. But he will play where he is most comfortable. We would love to have him."

The Panthers' principal owner is Albert Taubman, a Detroit real estate man reported to be among the league's wealthiest owners.

The Washington Federals of the USFL say they have not initiated discussion with any Redskins player because of potential negative reaction from fans here. Federals President Jim Gould said he would be interested in Riggins if the Redskins don't sign him. "We would consider signing Riggins if there was a clear and imminent danger of him leaving Washington," Gould said. "Then we'd have to call the league office and ask for a review."

In 13 games last season, including the playoffs, Riggins gained 1,163 yards in 313 carries, but it was four straight 100-yard-plus games in the playoffs that made this the best of his 11 NFL seasons.

In Milot's case, Bennett said Beathard twice has failed to make an offer by the date he promised. Bennett said Monday is the third date.

He said Milot had been offered "a moderate term" contract by the Stars for more than one year.