More than £70,000 has been saved by the Fire Authority after more firefighters than expected left the service - but the Fire Brigades Union said they should be replaced.
The £73,064 underspend was revealed as part of the authority's revised budget for spending to the end of December.
At a meeting of the authority on Monday, Margaret Jaworski, deputy treasurer, said the underspend had come about after more firefighters left the service than had been expected.
She said: "The savings have been found quicker than the government grants. People are leaving at a faster rate than expected, which is creating a year on year underspend."
Other changes include an analysis of the use of overtime, a vacancy within the support staff budget, reduction of retirements through ill health from four to two, and reductions in energy bills, rates, and vehicle repairs, maintenance and running costs.
There has also been a review of equipment, furniture and materials which has resulted in an underspend.
But John Gilliver, secretary of the Fire Brigades Union, said fire-fighters who had left should be replaced.
"The underspend for this financial year is £3.1m. Year on year they have had significant underspent. The reason why is because of additional people going. But they should replace them. The fire service is overstretched and it is a disgrace they are not recruiting.
"Investing in fire-fighters will save lives, that's what they should be investing in. It is the cheapest form of insurance, but we're saying put them back into the system.
"You can't depend on smoke alarms. All right, they wake people up but they don't get them out of the house and certainly don't put fires out. Only fire-fighters can do that."
He said the number of fire-fighters had decreased from 740 to just over 600, adding: "We think this is too low. What they are trying to do is privatise the fire service but we are not a business, we're a service to the public."
Fire authority chairman Jim Andrews said the 12 firefighters had not left, but had moved to other brigades.He added: "The finances are going to get worse and taking people on now means more people having to go and not leaving of their own accord with a pension.
"It could involve redundancies. As we have said there will be no wholetime firefighters made redundant. If they want to leave they will leave with their pension, or as the 12 people have, they will move to other brigades.
"Now there is also a £2m community budget which we are looking to spend on more smoke alarms for vulnerable people and different things in the four boroughs."