When are layoffs not really layoffs? When people volunteer to lose their jobs, and they could retire anyway.

It’s just that sort of volunteers the Portland School Department is seeking to encourage by offering bonuses from $10,000 up to a potential $20,000 for departmental workers to retire.

The offer is an expansion of inducements the system has made for the past five years, which officials say has generated significant savings.

What’s different this year is that Superintendent Jim Morse is telling employees who have worked for the system for at least 10 years and meet the mininum retirement age of either 60 or 62, depending on when they were hired, that the bonuses may or may not be around next year.

The other thing is that the potential bonus was capped last year at $10,000, so some workers could get twice that amount if they choose to leave by the end of the school year in June.

The spur for the offer is a continued projected revenue shortfall, which for the next budget year starting July 1 could be as much as $4 million.

Last year’s early-retirement bonuses led 43 eligible teachers, administrators and other workers to leave, saving the system $1.5 million because some were not replaced and other jobs were filled with lower-paid employees.

Administrators say about 200 of the department’s 1,140 workers are eligible for the new offer, which would pay a minimum of $10,000 or 30 percent of salary up to a maximum of $20,000, offered in four annual installments.

Eligible workers have until the end of the month to apply, but acceptance isn’t guaranteed. Morse said he only agreed to the plan on the condition that enough workers accepted it to guarantee “significant” savings, and that won’t be known until after Feb. 28.

It’s true that the offer doesn’t guarantee that only underperforming staff members will leave. Indeed, top teachers and other effective workers could just as easily take it as anyone else.

No, the goal here is far more direct — saving money in a way that doesn’t require the dismissal of people who like and need their jobs and want to keep them. Of course, that only works if enough people accept the offer.

If not, the $4 million shortfall is going to have to be handled in ways that will be less pleasing to staff. Which, of course, is the real incentive to grab the extra cash while it is on the table.