Is the accountant considered sinful and treacherous to the trust for not informing the management about his manager's transgressions in selling goods to non-compliant customers, knowing that the manager will pay from other customers' accounts, and what should he do? And is it permissible for an employee to invest his salary in an external business without the employer's knowledge if there was a verbal agreement not to work for another company?

Your friend is liable for what he sold to those prohibited from buying, due to his violation of his principal's valid condition, and because he defrauded and cheated by altering the account statements. As for you, you incur no sin if your disclosure of his matter would lead to greater harm, provided he repents, regrets, and guarantees what he squandered. What is required of you is to urge him to expedite payment and follow up with the creditors. As for your work in investing your money outside official working hours, there is no sin in it, and it does not fall within what you and your employer agreed upon.