What should I do with the money that was deposited into my account from the secondment, knowing that I did not go?

If the manager authorized to grant incentive bonuses (such as for a secondment or other remuneration without actual work) gives permission, then there is no harm in benefiting from it. However, if he is not authorized to do so and resorts to deception by disbursing it under the guise of an unreal secondment or non-existent overtime work, then it is not permissible for you to take the money. It must be returned to the responsible authority. If returning it is not possible, it should be spent on public welfare or given to the poor and needy. The manager's authority can be ascertained by reviewing the labor system regulations or by asking specialists.