The Washington Redskins finally reached an agreement with holdout cornerback Tory Nixon yesterday, but negotiations with veteran running back John Riggins apparently have reached an impasse, according to sources.

Riggins has indicated to the Redskins that their offer, believed to be worth close to $1 million for the 1985 season, is not acceptable, those sources said. Apparently, Riggins and his lawyer, Doug Woloshin, are asking for a one-year deal that may be worth as much as $1.5 million, sources said.

Owner Jack Kent Cooke, who negotiates directly with Riggins, declined to comment on the negotiations. Woloshin, speaking for Riggins, said he is "very optimistic" that a contract can be agreed upon, but he refused to discuss specifics. There has been no indication when the two sides will meet again.

Meanwhile, it was learned the Redskins have offered defensive tackle Dave Butz a two-year package worth $975,000: a $200,000 signing bonus, $325,000 salary in 1985 and $450,000 in 1986. Butz made $230,000 last season.

After meeting with Cooke Wednesday at the owner's Middleburg, Va., estate, Butz flew home to Belleville, Ill., to discuss the offer with his wife Candy. Butz was unavailable for comment yesterday.

It is unlikely Butz will arrive at the team's Carlisle, Pa., training camp soon. The Redskins apparently have given Butz, a 12-year veteran, no deadline to respond to their offer.

Nixon, the Redskins' top pick in the 1985 National Football League draft, apparently rejected the Redskins' offer of $925,000 for four years Wednesday night, then accepted one worth a reported $990,000, the value of the Redskins' original offer two weeks ago. He is expected to report today.

Nixon's agent, Larry Muno, said he was seeking $1,045,000 for his client, a figure apparently close to what players chosen near Nixon in the second round of the draft have been receiving. After the initial rejection, Redskins General Manager Bobby Beathard and Muno negotiated a $65,000 raise in Nixon's 1985 salary to make up the difference in the two Redskins offers, sources said.

Nixon reportedly will be paid a $200,000 signing bonus, $195,000 salary in 1985, $160,000 in 1986, $190,000 in 1987 and $225,000 in 1988. He also will receive $5,000 for reporting to training camp each season.

After practice at training camp yesterday, Coach Joe Gibbs said he was "happy that we signed Tory . . . He's missed a lot of work, but he's supposed to be smart, and we'll fix things up once he gets here."

Gibbs also said he does not plan any cuts before Saturday's scrimmage against the New York Jets at Easton, Pa.