Settle it before the agreement is signed and before travel is arranged. Terms that define who the client is, what the duties are, and how long the engagement runs are difficult to change once both parties have committed and a start date has been promised.
Work back from the client's expectations
Set the review against the client's needs: when the engagement is meant to begin, whether the scope is defined or open-ended, and how quickly the agreement can be amended. Where a US entity would need to be restructured, allow for corporate steps that take their own time. Keep the client informed that the structure is under review rather than promising a start date the arrangement may not support.
Freeze the application record only after the employer confirms the final currency, base pay, benefits, duties, worksite, and proposed start. A business need to start quickly does not guarantee TN admission or authorize work under an outdated account. Confirm the licensing sequence, employer start date, and room for correcting discrepancies. Keep early discussions about the arrangement clearly marked as preliminary so that circulating a draft internally is not later mistaken for a finalized structure. Save each version of the proposed terms with the date it was drafted, since a reviewer may need to see how the structure evolved rather than only the final version. Wait for that final version before treating any element as fixed, even where earlier drafts looked settled.