What should a Muslim do if he sees administrative corruption in his company, such as a colleague being marked present and receiving a salary while absent? Should he inform the general manager, or would that be considered slander and calumny? And would his commitment to silence be considered a silent devil?

It is obligatory for every Muslim to change evil according to their ability, based on the Prophet Muhammad's (peace be upon him) saying: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart—and that is the weakest of faith." What your colleague did, recording attendance while absent, is a lie and falsehood. You must denounce this evil, and doing so is not considered forbidden backbiting.