Social workers on temporary contracts are being offered sizeable 'golden hellos' and extra annual leave to stay with County Hall.
The expensive agency staff - believed to be mostly children's social workers - have been in high demand ever since children's services was revealed to be struggling with huge workloads by a damning inspection report in February 2013.
But the decision to pay some more than half the average wage of social workers in one go - - depending on experience and time at County Hall has angered Union bosses because it is so soon after promised pay increments across the board were scrapped in November 2016 due to lack of funds. Agency workers are also being offered the five extra days holiday that staff accrue after five years' service.
Penny Smith, secretary of Unison's Northamptonshire County branch, said: "It's about the message this sends to permanent workers.
"A cash-strapped authority who can't pay increments can suddenly find the money for this so soon afterwards. It's demoralising for the permanent staff
"Managers we've spoken to have been horrified that they have to implement this. It creates a two-tier system."
The lump sums range from £5,000 (for agency social workers who qualified less than a year and ago and have been with the council for under six months) to £15,000 (for someone with four or more years' experience who has been with the council for two years).
Unison says it recognises that the 'golden hellos' will not come close to the £1 million that had been set aside for pay rises for county council employees who had hit targets.
But it says favouring one group of employees over another is still a recipe for disaster.
Mrs Smith says a better solution to the desperate need for experienced social workers would be to make terms and conditions better for all, thereby attracting and retaining the high quality staff the county needs.
She said: "They could start by reinstating the former sick pay conditions [County Hall staff are no longer paid for first day of sick leave], resolving car parking situation, and being more generous with annual leave for all social workers.
"These are fairer solutions."
A county council spokeswoman said it wanted to create a more stable workforce, which will be cheaper in the long run than relying on cripplingly expensive agency workers.
She said: "We want to reduce our reliance on costly agency workers and enabling us to deliver better continuity of care for children and families in Northamptonshire.
"This is why we are actively encouraging our agency workers to consider joining us as permanent staff members with the incentive of an enhanced conversion bonus.
The council also revealed it is indeed looking at better rewards for all its social workers.
The spokeswoman said; “We greatly value the contribution of all our social workers, agency and permanent, and we are reviewing the total reward package for all staff as part of our move towards the new children’s services delivery vehicle, which will offer us greater flexibility in terms of ongoing pay and reward.”