Is a person considered sinful if they are responsible for organizing and preparing travel arrangements for government delegations, knowing that their manager will alter the original invoices to steal public funds, given that their role does not involve paying invoices or direct handling of money? And is it obligatory for them to leave this job?

There is no sin upon you for the corruption and theft committed by the manager, as long as you do not directly engage in those violations nor assist him in them. However, you must advise him, denounce his actions, and report his matter to the authorities, for the Prophet, peace and blessings be upon him, said: "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."