Shire Rothbart, vice president of the Michigan Panthers, confirmed reports today that the United States Football League team obtained conditional negotiating rights from the Boston Breakers to Redskin fullback John Riggins.

Rothbart, who also is the team treasurer, denied a report that the Panthers had already offered Riggins more than $1 million for two years. But he said he hoped to arrange a negotiating session including himself, General Manager James Spavital and Riggins "very soon."

Spavital, who as offensive backfield coach with the Jets in 1975 and 1976 coached Riggins, said in a telephone interview with WDVM-TV-9 last night the fullback "might be the Joe Namath of the USFL." George Dickson, the Panther backfield coach, coached Riggins when he was with the Redskins under Jack Pardee. He and Riggins were close friends.

"I've never met John Riggins and I've never talked to John Riggins but we hope to have an appointment with him soon," said Rothbart. "We intend to make him a very serious offer . . . He is very well-known. He can't do anything but contribute to our team and the league."

Asked if a contract with Riggins would have him play this year or next, Rothbart said, "I'd love to have him play this season. He's a free agent, and our understanding of that is that he would be available right away. If there's any question, though, of when he'd begin playing, well, then, we'd handle it."

Riggins was not available to comment today. After accepting an award presented by the Alexandria fire marshals this afternoon in Alexandria, he left the podium and drove off without speaking to reporters. In a brief speech, he said he knew nothing about the Michigan offer and added, "A guy would really have to be crazy to leave this place."

Redskins General Manager Bobby Beathard said he assumed Riggins would have other offers. "I figured that is going to happen to anybody and everybody all around the league who is a free agent. We certainly want John and plan to have John back. But I think that's going to be the same way with everybody on the Redskins."

Beathard said he met briefly last week with Riggins at Redskin Park. "It was just to find out when we could get together and (I) asked him if somebody was going to represent him and he said no. I asked him when we would get together, and he said he was going to come to see me this week."

Riggins was supposed to be at Redskin Park Monday for an interview with NFL Films, but never showed up.

Greg Campbell, an agent from Madison, Wis., who negotiated contracts for five Panther players, said a high-ranking Panthers official told him the team would pursue Riggins with a "bundle of money."

"I haven't spoken with Riggins yet and I have no idea how much money it will take to get him, but we'd like to have him," said Spavital. "I have a good rapport with John."

Riggins earned $330,000 with Washington last year, not including the $70,000 he earned in playoff money for the Redskins Super Bowl victory in January.

The Panthers' principal owner, Alfred Taubman, was unavailable for comment. Rothbart said Taubman was "right now sailing on the Nile."

Rothbart said Boston Breakers officials agreed to allow the Michigan team to negotiate with Riggins. In a computerized dispersal of veteran NFL players, Boston was assigned negotiating rights to Riggins. If the Panthers do sign Riggins, they must compensate the Breakers. The teams have not determined what compensation would be.

The Panthers are also hoping to announce soon the signing of Michigan wide receiver Anthony Carter. Rothbart and Carter's agent, Bob Woolf, have agreed tentatively to a salary, and Carter is reportedly taking a day or two to consider the deal.