Karen K. Johnson has told federal prosecutors that she received $20,000 to $25,000 in cash over more than two years in exchange for her refusal to testify before a federal grand jury investigating allegations of drug use by District Mayor Marion Barry and others, sources said yesterday.

Her brother, Jared Kinnon, also has described the alleged payments to prosecutors, the sources said. In a meeting with prosecutors that lasted more than two hours last week, the sources said, Kinnon said that he received money on Johnson's behalf while she served an eight-month jail term for contempt of court.

Johnson, a 35-year-old convicted cocaine dealer, told prosecutors that John B. Clyburn, a D.C. businessman, made payments to her and that she believes that businessman Roy Littlejohn and perhaps others contributed funds, the sources said. Johnson said she accepted money in installments -- usually of $200 to $300, with no payment of more than $1,000 -- at varying locations other than her house, the sources said.

Johnson's statements have not linked the payments to Barry, the sources said. Barry said last week, "If there were any payments, I don't know anything about that. Absolutely. Unequivocally."

In another development, attorneys for Barry yesterday modified their request for an order barring news leaks in the case. {Details, Page B1.}

Clyburn's attorney, Thomas Dyson, when asked last week whether Clyburn paid money to Johnson for any purpose, said that Clyburn "didn't pay Johnson any money, period. There were no payments of any type." Dyson could not be reached for comment last night.

Eugene M. Propper, Littlejohn's attorney, said last night, "This continuing allegation that he paid hush money is false. Mr. Littlejohn has not paid anybody directly or indirectly for testifying or not testifying. If {Johnson} is alleging to prosecutors or anybody else that Roy Littlejohn has paid her or anybody else hush money, it is not true."

Clyburn and Littlejohn are prominent D.C. contractors who have received millions of dollars in city government business. Barry has described Littlejohn as a political friend and Clyburn as a "very close personal friend."

G. Allen Dale, Johnson's attorney, said last night, "My client has directed me not to comment to the press."

Johnson's refusal to testify three years ago stymied a federal grand jury looking into possible drug use by Barry and other D.C. officials. Prosecutors hoped that testimony from Johnson, who pleaded guilty in June 1984 to charges of possession of and conspiracy to sell cocaine, would enable them to determine whether Barry committed perjury when he told the grand jury he never obtained cocaine from her.

Prosecutors intended to question Johnson, then an employe of the D.C. Office of Energy, about a secretly taped June 1983 conversation in which she told her former boyfriend, Franklin Law, that she had sold cocaine to Barry on 20 to 30 occasions. Law was acting as a government informant.

Barry has said he has never purchased or used cocaine.

Federal investigators uncovered evidence of the alleged payments to Johnson through wiretaps authorized as part of a 17-month undercover probe of D.C. government contracting that was disclosed May 22 by U.S. Attorney Joseph E. diGenova, the sources said. Sources said yesterday that Johnson used the money to support her young son while she was incarcerated and for general living expenses after she was released in July 1985.

The sources said that the periods between alleged installments became gradually longer and that the payments were said to have ended within the past 1 1/2 years.

Johnson, who will be on probation until July 17 on the cocaine charges, agreed to cooperate with prosecutors in part out of her desire to avoid returning to jail -- on a probation violation or on new charges -- and in part because of her desire to remain with her son, according to sources.

Also, investigators are examining whether Johnson's attorney in the drug case, John A. Shorter, played any role in the alleged payments, the sources said. Shorter is serving a 40-month sentence at the federal prison camp at Maxwell Air Force Base in Montgomery, Ala., for tax evasion. Prison officials said last week that Shorter refused a request to be interviewed.

In addition, prosecutors are investigating how Johnson obtained a job as a marketing consulting in 1985 for a job placement program called Liberation of Ex-Offenders Through Employment Opportunities, the sources said. Program officials said Johnson is paid about $19,000 a year.

David Molloy, executive director of the organization, said yesterday that "no one told me to hire her" or recommended her for the position. He said he interviewed others and chose Johnson in part because of her empathy for offenders and in part because of "the fact that she had gotten some publicity."

"The publicity implied that she would know people who could help the program in terms of donations" from private organizations, Molloy said. He said Johnson has exercised no influence over the organization's funding from the D.C. Department of Employment Services. The funding began in 1984 and now amounts to about $200,000 a year.

"Karen Johnson's presence has not benefited us," said Molloy. "Our budget from {the job agency} has gone down every year since she was hired." He added that he was "beginning to believe" that the connection with Johnson might have made private organizations more reluctant to give, fearing that "this might be said or that might be said."

Molloy rated Johnson's performance as "very, very good."

A spokesman for diGenova said last night that diGenova had no comment on the allegations of payments to Johnson.