Final answer:
A formal engagement letter is the best approach for recording an agreement with a specialist, as it clearly defines roles and expectations in a legally binding document, contrasting with the informal nature of verbal agreements.
Step-by-step explanation:
The best approach for recording an agreement with a specialist once the scope of work has been determined by the auditors is to use a formal engagement letter. This document should outline the responsibilities, scope, and expectations from both the auditors and the specialist, and provide a clear framework for the services to be provided. Compared to verbal agreements, which can be subject to misunderstandings and lack documentation, a formal engagement letter ensures that a legally binding understanding is established. Recording the agreement in the audit planning documents is also essential, but it serves as an internal control rather than a formal agreement with the specialist.