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Employers have no legal obligation to offer severance except in certain very limited circumstances involving mass layoffs which would be inapplicable here. Employers generally offer severance in exchange for a waiver of the departing employee's right to sue. The amount of severance will therefore be a function of the perceived litigation risk the employee presents. It is the employer's judgment call what to offer based on the circumstances.
When an employee quits or retires, they would typically have no basis to sue their employer because they are voluntarily initiating their own unemployment. This is why you are being offered less than other employees who were laid off. The employer does not perceive any serious litigation risk posed by your departure.
The fact you may have been a "whistleblower" would not matter for a couple reasons. First, unless you reported this use of funds to a government agency (i.e. the IRS), you would not typically be covered under any whistleblower statute. Second, even if you were covered and this whistleblowing was protected conduct, you still cannot link that protected conduct to any adverse employment action because you are retiring, not being forced out against your will. If you were being laid off, you might be able to argue that the excuse of a "layoff" is really just a pretense for retaliating against you because you were a whistleblower. But this argument is a non-starter is you are voluntarily leaving your job. You can't argue that your employer is retaliating against you for something you have voluntarily decided to do.
So, perhaps it would be best for you to continue working until you reach full retirement or until you are laid off, in which case your employer might be more inclined to offer severance. Of course, how you choose to proceed is a decision only you can make.
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