The Welsh Assembly has not yet found time to approve funds to support the package - a fact described as "crazy" and "a tragedy" by Alun Jones, director of the National Association of Head Teachers in Wales.
"Heads are going to suffer at least an extra term's stress as a result of this," he said. "They'll have to take part in training days and initial assessments of teachers, when the idea of the offer was that they wouldn't have to."
In England, where local authorities had to submit bids for top-up funding by early December, more than 300 heads are taking early retirement. Top-up funding is needed to ensure those who retire early and thuspay into their pension fund for fewer years do not lose out on future payments.
But in Wales, no date has been fixed for the assembly to debate the issue so local authorities cannot yet invite heads to apply.
Mr Jones said that the assembly had only needed to act as a funnel for the money but had failed to act."We don't mind lagging a day or two behind but to lag months behind is a tragedy," he added.
Ministers have set aside pound;10 million for the early retirement of heads in England and Wales between the ages of 55 and 59 "who are not able to carry forward the major changes envisaged". Those who take early retirement will see their individual pension funds boosted by up to pound;50,000 each, with half of that sum coming from the local authority and half from special government grants.