CRA states that Canadian domestic sourcing rules apply in determining whether a Canadian corporate donor to a U.S. charity has US-source income for Treaty purposes

A gift made by a Canadian-resident corporation to a U.S. resident charity that also would qualify as a Canadian registered charity if it were resident in Canada is deemed by Art. XXI, para.7 of the Canada-U.S. Treaty to be a “gift to a registered charity” for purposes of the Act, subject to an assumption that the corporation’s income only includes its income arising in the U.S. considers that the Canadian domestic sourcing rules should be used in determining whether income has a U.S. source for these purposes so that, for example, the policy in Folio S5-F2-C1, paras. 1.62 to 1.65 would apply in determining whether a capital gain had a U.S. source.

The 75% income limitation (re income from U.S. sources) and the five year carryover rule in s. 110.1(1)(a) also apply for these purposes.

Neal Armstrong. Summary of 2015 TEI Roundtable, Q.7, 2015-0614251C6 under Treaties, Art. 21.