CRA indicates that the $10,000 safe harbour for Canadian employment income of U.S. resident is applied on calendar year basis even where she was a part-year resident
Art. XV, 2(a) of the Canada-U.S. Treaty provides a safe harbour, from the application of a general rule permitting Canada to tax the employment income of a U.S. resident derived from "an" employment exercised in Canada, where the remuneration from such remuneration does not exceed Cdn.$10,000. CRA considers that this safe harbour is applied on a calendar year basis so that if a Canadian resident becomes a U.S. resident on, say, December 1, but has income from the exercise of employment with his Canadian employer in December of under $10,000, he will be unable to access the safe harbour.
Given that it would be unusual for such an employee to continue performing his duties of employment in Canada for the same legal entity, this position may mostly be relevant where the individual receives some post-departure employment benefits from Canada.
Neal Armstrong. Summary of 10 February 2015 T.I. 2013-0484501E5 under Treaties – Art. 15.