Three weeks ago Michael Hershman watched the last big case in his government career come to an end: As Hershman's hidden videotape machine spun away in a Wisconsin Avenue hotel room, a senior official with the Agency for International Development, George C. Warner, allegedly accepted a $9,000 payoff from a rice seed brokerage firm.

Hershman's recording of the transaction in a Georgetown Holiday Inn room -- which allegedly occurred so Warner would not take away a contract from a Thai firm that supplied rice for America's Cambodia relief program -- has now become evidence for the courtroom. And with that notch on his belt, Hershman is leaving his job as AID deputy auditor general to team with Washington private eye Dick Bast.

"I'm into financial types of investigations," says Hershman, 35, whose low-key personality contrasts with Bast's sometimes boasting, overwhelming manner. "I'd hope to attract as clients the multinational corporations who are troubled with fraud and embezzlement."

Hershman's new boss is one of the capital city's legendary sleuths. Bast recently earned a startling $321,000 fee working for the Church of Scientology in an unusual effort to force U.S. District Judge Charles R. Richey to remove himself from a case involving the church.

Bast once smashed in a hotel door at a posh Caribbean island retreat, the better to photograph Barbara Howar, then married, in bed with an official in Lyndon Johnson's White House. The official was also married, but not to Howar.

While Bast was gaining a reputation as a high-priced fast-shooter, Hershman was beginning a career as a government sleuth.

After two years of college near his home town on Long Island, Hershman spent three years working for military intelligence as a counterterrorist expert in southern Germany. Then, as he pursued a degree in criminal justice at the State University of New York, he became an investigator for the city and state of New York. Between 1970 and 1973, Hershman investigated judicial, police and correctional system corruption, first on behalf of the Knapp Commission, later with Maruice Nadjari's New York State Special Corruption Prosecutor's Office.

In the spring of 1973 Hershman was named senior staff investigator of Sam Ervin's Senate Watergate committee. His specialty was electronic surveillance, which led to his next job as executive director of the National Wiretap Commission. A stint as chief investigator of the Federal Election Commission followed in 1975. Then he headed the probe of Korean influence in Washington run by Rep. Donald M. Fraser (D-Minn.) that uncovered shakedowns of American businessmen in Korea, spotlighted early executive branch knowledge of Tongsun Park's largess involving congressmen in Washington, and produced the former head of the Korean CIA as a witness.

In 1978 AID asked Hershman to rebuild their audit and investigative capability because, says Hershman, "they said they were concerned their staff was not adequate to watch $4 to $5 billion in assistance per year in some of the most risk-prone environments in the world."

Hershman says he never intended to make a career in government, and the offer to join Bast came at the right time. Hershman leaves AID for a $35,000 salary from Bast, a new Cadillac and an option to buy about a third of Bast's company, International Investigations, Inc. Next month Bast checks into a hospital for a complicated heart operation, and he says he wanted someone like Hershman to stay with his bizarre business. It's a partnership that should worry everyone who takes payoffs in hotel rooms anywhere.