The reward had been offered for information leading to the arrest and conviction of Dorner, who police alleged went on a killing rampage last month, the Los Angeles Times reported Friday.
The Peace Officers Research Association of California pulled its $50,000 commitment to the fund because the reward criteria had changed, union President Ron Cottingham said.
Cottingham said the reward was for information leading to the arrest and conviction of Dorner, a disgruntled former Los Angeles police officer who died from an apparent self-inflicted gunshot wound during a standoff with police.
Riverside city officials pulled their $100,000 pledge to the fund this week.
The head of the LA Police Protective League said his group is considering withdrawing its pledge.
LAPD Deputy Chief Kirk Albanese has said it would be "disingenuous" for donors to withdraw their pledges simply because Dorner did not go to trial.
Two claims have been filed for the reward by people who encountered Dorner during a massive manhunt.