ROYAL OAK, Mich. -- Royal Oak police are warning about a fake donation scam that dupes businesses into thinking they are donating to police.

"It will cause the public to lose trust in the police department," said Royal Oak Police Sgt. Robert Reilly. "It will ruin our reputation for being professional and being honest."

Police said a man is calling doctors' offices and claiming he is a city police sergeant collecting money for an anti-drug campaign.

According to the victims, a woman stops by the offices and collects the donations.

She is described as a medium to dark-skinned black female, 35 to 40 years old, about 5 feet 7 inches tall with short hair, heavy build and a gold tooth.

All of the victims told police the man uses different names on the phone.

Police said when the victims wrote out a check, the check was altered with a higher amount.

"They alter the check to an individual and then they alter the amount, and they've been successfully cashing some of these checks," said Reilly.

There have been at least three victims and a check for $25 dollars was altered to $2,500.

Anyone with information should call Royal Oak police at 248-246-3456.