A D.C. Superior Court judge yesterday awarded $400,000 to the family of a 30-year-old Washington businessman who was shot to death in 1978 while acting as a police decoy during an armed robbery attempt.

In holding the city liable for the death of Elton Leon Grant, Judge John D. Fauntleroy took the unusal step of taking over the case from a jury that was deadlocked after two days of deliberation after deciding to render a decision in the case himself.

The lawsuit charged the city government with negligence and failing to devise a plan that would adequately protect Grant, who was shot once in the throat and robbed by three men while a dozen police officers looked on. The incident took place as Grant withdrew his weekly payroll money of the Park Road branch of the Riggs National Bank on July 21, 1978.

Grant had learned of a plan to rob him one week before the shooting, according to the suit, and had alerted D.C. police officers at the 4th District. The decoy plan was then arranged, according to the suit.

The suit also alleged that Grant, who owned a flooring and refinishing firm at 7325 Georgia Ave. NW, expressed fear about the decoy plan but participated in it "at the insistence and urging" of city police officers.

"It was their plan, their assurances," said Samuel Shapiro, attorney for Grant's family in arguments to the jury.

Asistant D.C. Corporation Counsel Michael E. Zielinski argued that Grant had not been forced to participate.

"Nobody coerced anybody to do anything," Zielinski said. "Nobody made anybody do anything . . . The risk was there. It was very clear from the evidence that Mr. Grant knew it."

The trial, which lasted about two weeks, almost ended in a mistrial as the jury reported to Faunatleroy Monday afternoon that it was deadlocked. Fauntleroy initially decided to permit the jury to deliberate for another day. fBut yesterday morning, he reversed himself after attorneys for both sides agreed to let him decide the case.

There was also a request by attorney's that Fauntleroy's final determination and damages award be kept secret from the public. The judge sealed the case records during most of the day, and a Washington Post reporter was barred from the courtroom for a short time by a court clerk while the judge and attorneys discussed disposition of the case. The reporter later was admitted and the verdict and award were announced.

The slain Grant's assailants, Arthur Swarn, Melvin Stewart and Leonard J. Anderson, all pleaded guilty to second-degree murder and other charges. India Stroman, Grant's former secretary, pleaded guilty to conspiracy and armed robbery.

According to evidence presented in the criminal case, Stroman, who worked for Grant until a week before the shooting, met several times with Anderson, who was her boyfriend, and Stewart and Swarn to plan the robbery.

Grant learned of the plan to rob him from a friend, however, and went to police. He was outfitted with a bullet-proof vest and went about his weekly routine of withdrawing payroll money from the bank. Under the plan, police officers staked out nearby were to arrest the suspects while the robbery was in progress.

Shapiro represented Grant's widow, Ruth, and the Grant's two daughters.