CUMBERLAND — An apparent error made by negotiators for the Cumberland teachers union three years ago means the town will not have to come up with an additional $1 million for salaries, an arbitrator says.

But the union is considering a court appeal among other options, said union president Roderick McGarry.

“We were very disappointed with the ruling,” McGarry said. He added: “We’re not ruling anything out at this point.”

At issue is the interpretation of which salary increases correspond to a schedule of “steps” in the contract, which ends Aug. 31.

Union members ratified the three-year deal, which had involved a mediator and various counterproposals, on Sept. 1, 2006, after the old contract expired. The School Committee also approved the contract.

But when some teachers received paychecks in the contract’s first year with amounts below the figures presented by the union’s negotiators, they complained.

According to the arbitrator’s report, the union reviewed its presentation “and realized that an error was made” for teachers in salary steps five through eight. Union and School Committee representatives met to try to resolve differences, with spreadsheets of how salaries would correspond to steps, but different interpretations remained.

In the contract’s second year, “numerous” teachers complained, the arbitrator said, because they believed paychecks issued on Sept. 14, 2007, had them on a salary schedule at one step lower than they should have been. The union filed a grievance that month. The matter ended up in arbitration.

After a nine-month review, arbitrator Lawrence E. Katz said the union negotiators’ presentation to union members had inadvertently listed salaries a step higher than they should have been for teachers in steps five through eight.

Katz’ arbitration decision says negotiators from both sides erred in not having clarity, sometimes relying on information that was not in writing.

But the union’s view of the step system, Katz said in his 75-page ruling, “would produce a disproportionately high pay increase for the teachers.”

If the arbitrator’s decision is the last word, the town would not have to pay two years of additional salary increases that would have come to more than $500,000 per year.

“It was critical that the School Department prevailed in this arbitration,” Schools Supt. Donna Morelle said.

mmckinne@projo.com