In April, the company agreed to pay $10.5 million to the government to settle the charges. That put Mr. Marchese in line to receive as much as a quarter of that sum, or $2.8 million, as his reward for blowing the whistle.

But the Justice Department has had a change of heart about Mr. Marchese and is asking a federal judge to award him nothing. The government now contends that he was the mastermind of a much bigger scheme within the company than the ones on which he blew the whistle.

“Giving Marchese a share of the settlement proceeds would merely serve to reward Marchese’s egregious wrongdoing,” federal prosecutors in Seattle stated in a court filing this month.

Whistle-blower lawsuits like Mr. Marchese’s, or so-called qui tam actions, date to the Civil War. That is when Congress, reacting to fraud by companies selling defective supplies to the Union Army, passed the False Claims Act, giving citizens the right to sue on the government’s behalf and be rewarded for acting as an informant.

The concept, as one lawmaker described it at the time, was to set “a rogue to catch a rogue.”

But if the government believes that a whistle-blower was also a “planner or initiator” of the fraud, it can argue that the reward should be reduced, even down to zero. That is what the Justice Department is now saying about Mr. Marchese, 38, who was a sales representative at Cell Therapeutics, a company based in Seattle.