Last year the government agreed to increase teachers' salaries according to their experience.
But the Ministry of Education failed to factor the pay increase into its budget for 2013.
The National Teachers Association says it may cost the government more than $30 million to cover the agreement.
The union says that after talks with the government, it has agreed to extend school holidays by a week to give officials time to find the funding.
The general secretary of the National Teachers Association, Johnley Hatimoana, told Pacific Beat improvements in conditions were approved by cabinet on February 9 last year.
"I'm so surprised that the government did not factor those into their annual budget last year," he said.
"Very recently we realised that even the 2013 budget was not factored in.
"So that scheme of service . . . we have agreed on - but the government has failed to budget for that restructuring exercise.
" And now the government is saying it is their fault, so we will have to give them time."
The union demanded the ministry make its costings within the next week or two.
Whether the issue is resolved within a week "depends entirely on the Ministry of Education and the Ministry of Finance," Mr Hatimoana said.
"We have agreed to that and even if the Ministry of Education do not complete that exercise by the end of next week, they may have to extend another week.
"As soon as they pay the arrears, then teachers will return to work and we will cover these two weeks somehow during the course of the year."