Here’s the fine print of the last-minute financial compromise that staved off unprecedented layoffs for hundreds of Miami-Dade police and other county employees: It’s only good for this fiscal year.

That means the county will have a big hole to fill in next year’s budget to make up for the one-time fix, setting up another union conflict — soon.

When Mayor Carlos Gimenez crafts his spending plan for 2012-13, he will not be able to count on unionized county employees putting an additional 4 percent of their base pay toward health care, bringing their total contribution to 9 percent, or on tapping a health-insurance trust fund for $10 million.

Together, the two measures amounted to $65 million to balance this year’s budget.County commissioners, by one vote, approved the controversial concession and trust-fund raid last month. The decision saved the jobs of hundreds of county workers, including 118 police officers and 17 corrections workers.

But unionized employees, who have to take a procedural vote on the extra 4-percent giveback, are almost certainly going to reject the concession. The result: Instead of the measure becoming a part of the unions’ three-year contracts, the county will only be able to impose the additional 4-percent contribution through Sept. 30, the end of the fiscal year.

Gimenez, who has been vilified by the Police Benevolent Association, sounded an early warning in a recent memo to commissioners.

“Budgeted reductions to personnel costs — either through concessions or layoffs — must occur,” Gimenez wrote in a 2012-13 budget priorities letter two days after they imposed the 4-percent concession on Jan. 24.

The spending plan that commissioners approved last year lowered the property-tax rate and was designed as a two-year plan to maintain county services while holding the line on taxes. It relied on significant employee concessions — or layoffs — that also would be needed next year.

The mayor says now, however, that his administration aims to produce enough cost savings from his much-ballyhooed county reorganization to avoid any layoffs and minimize union concessions, though some givebacks will be necessary.

“We’re looking to save money with design changes to the health-insurance plan,” Gimenez told The Miami Herald Friday. “Hopefully, labor unions will be on board with that.”

Last year, Gimenez unsuccessfully proposed a 5-percent healthcare contribution for at least two years which, combined with freezing vacant positions, would have amounted to a $65 million budget savings. Without all that money for next year, the budget will face a similar shortfall, the mayor said.

The $65 million hole could get bigger or smaller, depending on how much money Miami-Dade takes in from property taxes and the size of savings realized from the reorganization. The mayor has pledged to speed up the reorganization to unveil more of his plan over the next couple of months.

Commissioners approved a 2011-12 budget last year predicated on $239 million in concessions from the county’s 10 employee bargaining units. The unions agreed to steep cutbacks but hit an impasse over the additional healthcare contribution.

With the specter of another union battle looming, Gimenez’s hard-fought win in obtaining the 4-percent compromise will be short-lived. A second round will coincide with Gimenez’s re-election campaign, where his chief rival will likely be Commission Chairman Joe Martinez, a former police officer who voted against the concession.

County unions typically get involved in mayoral elections, offering hands-on-deck, organization and campaign contributions to their preferred candidate.

“I think there’s going to be backlash” against Gimenez, said Greg Blackman, president of the Government Supervisors Association of Florida OPEIU Local 100, which represents nearly 4,600 professional employees and supervisors. “The employees are upset.”

Blackman’s GSAF union rejected the 4-percent concession Thursday by a vote of 94 percent to 6 percent. Five more unions have votes pending. (Two unions, representing aviation workers and firefighters, avoided the 4-percent concession altogether by agreeing to other givebacks.)

The impasse with the unions arose when Gimenez’s administration proposed the 5-percent healthcare contribution. Commissioners rejected that plan and took up the issue again only after the mayor vetoed their original decision.

It is unclear if the police union, which sued the county over its handling of the anticipated layoffs, will put the 4-percent concession before its 5,400 police and corrections members. John Rivera, president of the PBA, said the union believes Gimenez did not have the authority to veto the commission’s initial vote.

Commissioner Barbara Jordan, who negotiated the 4-percent compromise with Gimenez and swung the 7-6 vote in the mayor’s favor, urged county workers from the dais to reject extending the concession beyond this year.

“I do not support, nor will I support in the future, any money coming from employees,” she said. Of the employees, she added: “I’m sure they’ll know how to vote.”

Commissioner Audrey Edmonson, who favored the compromise, said from the dais that she worried non-union county workers would still have to pay the 5-percent healthcare contribution Gimenez imposed in July.

Those employees indeed will pay the higher percentage through the end of this fiscal year, spokeswoman Suzy Trutie said , though for next year, the mayor told commissioners he may consider a lower contribution from employees who make lower salaries.

About 2,800 workers out of the county’s approximately 26,600 employees are not union members.

Knowing he will need commissioners to sign off on his future reorganization and union-contract plans to avoid a second, potentially costly political clash, Gimenez sent board members a memo the day after they approved the compromise striking a conciliatory tone.

The budget is complicated, he wrote, and this year, “our debates have been frank, emotions have run high at times and we have not always agreed on the best course of action.“Ultimately, though, we have come together and fulfilled our obligation to balance the budget, and more importantly, we have joined together to do what I believe is in the best interest of that public that each of us has pledged to serve.”