The letter from the president of the Washington Teachers' Union to city finance officials last April included a one-sentence request: Withhold $160 in union dues from special paychecks sent to teachers as part of a new contract.

Left unsaid was that the $160 was 10 times the amount called for under the union's formula for calculating membership dues. But neither school administrators nor city officials noticed the discrepancy. They agreed to the request and forwarded $832,480 to the teachers union.

A teacher's complaint about the dues overcharge led to an investigation by the union's parent, the American Federation of Teachers, and ultimately to allegations by the FBI that the union's president and others misspent more than $2 million in union funds.

Now, teachers are asking why the school system and the city's office of chief financial officer, which issues teacher paychecks, did not verify the dues amount listed in then-union President Barbara A. Bullock's letter.

The error was made on retroactive pay that teachers received under the new union contract. The union is reimbursing members, but it had to take out a loan -- which the members will have to repay with interest.

Some teachers maintain that the city should compensate the 5,000-member union for the excess withholding, and several labor lawyers said the city may be at least partially liable for those funds.

"To me it was an obvious error," said Arthur Siebens, a teacher at Wilson Senior High who previously was that school's building representative for the union. "Anybody just scanning that would notice that the union dues entry was too big -- disproportionately large, unprecedentedly huge. Somebody should have been looking at that extremely carefully."

Bullock's request to withhold $160 in dues from each of the special checks was a vast departure from normal practice. The city usually withholds $24.75 in dues from every union member's paycheck and slightly less for nonmembers, who are assessed an administrative fee.

The special checks -- which teachers received in June to cover a 5 percent pay raise that was retroactive to the fall of 2001 -- were for less than a normal paycheck. For example, a teacher making $40,000 a year would get a regular paycheck of $1,538 every other week. The special check for the full retroactive amount would have been about $1,250.

According to the AFT, the amount withheld from the special checks should have been $16 under the local union's bylaws.

City finance officials said they had no policy requiring them to verify the amount listed in Bullock's letter. School officials said double-checking such information was not their job.

School officials "don't have anything to do with the amount. It's between the union and its members," said Louis J. Erste, the chief operating officer for the school system.

Mary Leary, director of labor relations and collective bargaining for the District government, said her office insists on seeing union bylaws and minutes of union meetings when receiving a request such as Bullock's.

"I would have asked for the authority for that," said Leary, whose office does not deal with the school system. "If you're going to withhold monies from employees' salaries for union dues, it should be the appropriate amount."

At the time Bullock wrote the letter, the union was hundreds of thousands of dollars behind in its dues payments to the AFT. Sources familiar with the situation said Bullock asked for the excessive withholding so that the local union could pay those overdue bills. Bullock has not returned reporters' phone calls.

E-mails provided under the Freedom of Information Act show that after receiving Bullock's letter, school and city officials began asking how much to deduct for teachers whose back pay would be small because of their employment history.

The school system's general counsel, Veleter Mazyck, wrote an e-mail saying Bullock had instructed her not to deduct any money from checks of less than $1,000.

The school system's chief financial officer, Robert A. Morales, wrote a memo in May to the head of the city's pay and retirement office authorizing the $160 deduction.

Morales said he did not realize at the time that the amount was too high. "I didn't have anything to tell me that this was a wrong amount," he said.

The money was deducted from paychecks issued June 14. About the same time, Bullock paid a similar amount of money -- in several installments -- to the AFT.

Some labor lawyers said the city could be found liable for the money because school and city officials should have noted the sizable difference between the normal dues withholding and the $160 request.

"I would feel comfortable arguing that they're liable for it," said Woodley B. Osborne, a District labor lawyer.

Other lawyers said that such a claim might be difficult to make. Mazyck said the school system has no legal liability in the matter.

Alex Wohl, a spokesman for the AFT, said the parent union would "look into" whether the city might be liable for those funds.

Bullock initially blamed the overcharge on the school system, Wohl said, but later said it was the union's mistake.

Once teachers discovered how much they had been billed for dues, they flooded the school system and the union with complaints. At least one teacher complained to the AFT, which prompted the parent union to begin an audit.

The AFT turned over its preliminary findings to authorities. Last month, the FBI filed an affidavit in U.S. District Court accusing Bullock, her former assistant Gwendolyn M. Hemphill, former union treasurer James O. Baxter II and others of diverting more than $2 million in union funds for personal benefit. All those implicated in the probe have left the union's employment.

An AFT audit released earlier this month said the misspending totaled more than $5 million.

Veleter Mazyck, school system general counsel, said the schools have no legal liability for the dues overcharge by the teachers union.Louis J. Erste, chief operating officer for D.C. schools, said the amount of dues withheld is a matter between the union and its members.Then-union President Barbara A. Bullock's letter led to controversy.