When dealing with government make sure that the act and the government actor is authorised or you will be left without rights.
An employee of the SA National Defence Force entered into an employment contract with the Surgeon General which entitled him to certain benefits he was not entitled to according to the applicable law. When the Defence Force sought to recover the overpaid benefits he went to court relying on a contractual right and a right under the Constitution to fair labour practices.
The court in Opperman v The Minister of Defence and Military Veterans found that there was no legal basis in the Defence Act for the claimant to be paid the additional benefits. The additional remuneration received as a result of the erroneous classification of his rank was therefore wrongfully granted remuneration. Where public funds are involved, unauthorised remuneration means that it has been made without a lawful basis and the overpayment must be recovered. The department was thus duty-bound to recover the amount from him once the mistake was detected and the employee had no contractual or other right to assert.
The legal fees in this matter probably cost the claimant more than the amount he should have refunded in the first place.