Continuing Canadian activity does not itself authorize U.S. work.
Assess the actual location and permission
Identify the proposed tasks and relevant process. A business schedule or foreign payroll arrangement is not an admission or authorization decision.
A Canadian citizen with a complete package can seek admission at a port of entry as soon as the letter and qualification evidence are ready. A Canadian citizen may generally seek TN admission at a port of entry, or the employer may file Form I-129 where suitable, but neither path is completed merely because funds arrived. TN admission may be granted for up to three years and may be renewed if the requirements continue to be met, so plan around the confirmed admission rather than the planting target.
The safe sequence starts with a written confirmation that the duties fit a listed profession and the credential limb is satisfied; for a diploma-plus-experience case that includes collecting the employer letters, which can take weeks if a former employer is slow.