What should an accountant do with excess money that comes into his possession, the owner of which is unknown, knowing that informing the authorities might expose him to accusations of theft?

If there is an increase in your salary due to a calculation error, you must search for its rightful owner. If the owner cannot be identified, keep the money for a period. If their identity remains unknown and you are certain it does not belong to the administration, then give it away as charity with the intention that its reward goes to the owner. If you later come to know the owner, give them the choice: either they approve the charity and receive its reward, or you pay them their money. If the money belongs to the company, you must return it to them by any possible means, without necessarily informing the administration.