Keep the actual identity and document status clear and align final records before use.
Avoid an invented document number
The employer should not guess new passport details. Review applicable travel and filing requirements against the documents actually issued rather than promise a start date from the renewal application alone.
The employer can draft the support letter while renewal is underway, using the applicant’s legal name and omitting an unissued passport number. Before submission or travel, reconcile the final passport with the letter, degree and any name-link documents, and confirm that the document will be valid for the request. TN requires Canadian or Mexican citizenship, prearranged employment in a listed USMCA profession, and that profession’s qualification; a pending renewal proves none of those elements. U.S. self-employment remains unavailable. A Canadian citizen may apply at a port of entry or an employer may file Form I-129, but the parties should not promise work before authorization.
Hypothetical example: A Sainte-Catherine-de-Hatley computer systems analyst receives an offer while her renewed passport is still being produced. The logistics company prepares a letter describing systems-analysis duties, salary, temporary period and supervision, then holds the application packet until she can verify the passport details. She gathers her computing degree and legal-name evidence in parallel. The employer avoids booking a fixed start around an estimated delivery date and prepares a contingency for delayed travel. Guessing the new passport number would create a needless inconsistency; waiting to confirm it does not require inventing a different employment history.