Ottawa's police services board has refused to cover the $540,000 bill for Sgt. Steve Desjourdy's defence so his union is taking the city to arbitration.

Desjourdy was acquitted of a sexual assault charge in April connected to a Sept. 2008 incident inside the Ottawa police cellblock.

Defence lawyer Michael Edelson and his team represented the officer and soon after, it handed the police services board the lofty bill.

Eli El-Chantiry, chairman of the police services board, claimed the bill lacked specific details for services rendered.

He also told CBC News the bill would have never passed through the city's legal department.

But in what El-Chantiry called a "clerical error," a cheque was issued to cover all legal fees. Then five days later, a stop payment was issued for the cheque.

Matt Skof, president of the Ottawa Police Association, called the move "unprofessional."

Letter sent to all police officers

He has since sent a letter to all police officers informing them the union will take the police services board to the arbitration to settle the issue.

Skof argued the legal fees debate is more than a clerical error, but instead centres on a disagreement regarding the collective bargaining agreement between the police service and officers.

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Matt Skof of the Ottawa Police Association says the police services board has been unprofessional in its dealings with Desjourdy's legal fees. (CBC)

Skof said the police services board has challenged the language of the indemnification clause, which details the extent of protection Desjourdy receives from legal issues related to his work.

Skof claimed the board wanted to link indemnification to the result of the pending discreditable conduct charge against Desjourdy under the Police Services Act.

In other words, the city would not cover costs if Desjourdy were found guilty. They would if he was let off.

El-Chantiry would not comment on that claim.

Desjourdy is facing this latest charge related to the same cellblock incident.