Is it considered a sin for an employee to amend his salary from 1000 to 1200 Riyals, sign the new contract draft instead of the HR manager, then present it to the General Manager who approves it, after which a copy is sent to the HR manager who in turn approves it and does not comment?

It is not permissible for the questioner to sign on behalf of another, as this constitutes deception, forgery, and falsehood. He must repent to Allah Almighty for this, and the knowledge and consent of the person who was lied about do not remove this sin. As for the new salary that the general manager signed, and the human resources manager knew about and approved, it is permissible, and it has no connection to the obligation of repenting from the sin of deception and forgery.