Is the money taken for a secondment that the employee does not attend lawful, knowing that his manager is aware of it and approves of it?

An employee is considered a private hire (ajir khass), who is entitled to wages if he avails himself for work, even if there is no work to be done, or if the employer or his representative grants him permission [to be absent]. So, if your manager is authorized by the employer to exempt you [from work] and grant you the salary, then there is no harm. Otherwise, he is not permitted to do so, and it is not permissible for the employee to leave work based on permission from an unauthorized person.