Explanation
The two actions that are required to achieve the requirement are setting up External Object to use OAuth to connect to the Accounting system and restricting access to data in the accounting system. OAuth is an authentication protocol that allows Salesforce to securely access data from an external system without storing the username and password of the external system user3. Restricting access to data in the accounting system is a way to ensure that only authorized users can view the invoice records from the external source. Creating an owner based sharing rule to grant visibility to the Invoice object and granting access to the External Object to only the Account Manager profile are not actions that can ensure that a user only accesses invoice records the user is authorized to see.