Federal prosecutors asked U.S. District Judge Stanley S. Harris yesterday to dismiss cocaine distribution charges against D.C. contractor Herbert (Dusty) Young, saying that a key witness would not be available for the trial, which was to begin Monday.

Sources said that FBI agent Allen Crawford, who allegedly made two cocaine purchases from Young, is involved in a major undercover drug investigation and would not be able to testify.

The government's motion, filed yesterday by Assistant U.S. Attorney Rhonda C. Fields, asked Harris to dismiss the indictment without prejudice, which means that the charges could be refiled later. Harris, who canceled the trial, has not yet ruled on the dismissal request.

Young, who had intended to represent himself during the trial, could not be reached for comment yesterday. Defense lawyer Ernest W. McIntosh Jr., who was appointed by Harris to assist Young, said he will oppose giving the government the option of refiling the charges. "Mr. Young has had a cloud over his head since August," said McIntosh, adding that the matter should be resolved.

Fields said in the government's motion that she did not know when her witness would be available to testify. Because of the time that has already elapsed since Young's indictment, the government would be unable to seek any significant additional delays under the federal speedy trial act. The act says a defendant must must go to trial within 70 days of indictment, with some exceptions considered.

McIntosh said Young believed the government's decision not to go to trial next week is related to a defense request to prosecutors for the FBI to provide its so-called informant's file on Young. Young has maintained that he was charged in retaliation for his refusal to cooperate with the FBI's ongoing probe of city contracting practices.

Sources have said that Young at times had provided information to FBI agents about alleged contracting improprieties in Mayor Marion Barry's administration, and at the same time, Young allegedly funneled information to Barry's associates about the FBI's investigations.

Young, 58, of 2518 Q St. SE, was arrested Aug. 11 and charged with using a telephone to facilitate a drug transactions. He was indicted 10 days later on that charge and two cocaine distribution charges.

According to the indictment and the affidavit in support of an arrest warrant, Young sold cocaine to Crawford on Oct. 14, Oct. 16 and Nov. 7, 1986. However, the third sale allegedly took place in Maryland and Young was charged here only with making the telephone call that preceded the transactions.

G. Allen Dale, the defense attorney appointed by U.S. Magistrate Jean F. Dwyer to represent Young, asked to withdraw from the case in early December, citing "irreconcilable differences" between him and his client.

Harris allowed Dale to withdraw and McIntosh was appointed to represent Young. But Young later told the judge he would not accept McIntosh. Harris ruled that McIntosh would serve as an "advisory counsel" to Young and then delayed the trial, which was to begin Jan. 6, until Monday to allow Young and McIntosh to ready their defense.