Is it permissible to receive a hidden payment for performing the work of an absent colleague, which has been entirely imposed upon me without my consent or compensation, especially since the employer does not acknowledge this right and it affects my well-being and my family's life?

The employer does not have the right to compel the employee to perform tasks beyond what is agreed upon in the contract. The employee has the right to refuse overtime work. However, if the employee accepts it with the knowledge that there will be no compensation, he is considered a volunteer and has no right to demand compensation.