Alberta Gas Trunk Line Co. Ltd. v. Minister of National Revenue, [1972] S.C.R. 498, 71 DTC 5403, [1971] CTC 723 -- summary under Foreign Exchange

The taxpayer borrowed U.S. $67,000,000 for the construction of capital assets and in the year of borrowing sought to deduct $358,828 from its income, being the difference between $67,000,000 and the equivalent in Canadian dollars at the time of borrowing of U.S. $67,000,000. The supposed loss of $358,828 was not deductible because whether or not any loss would be sustained by the taxpayer would depend upon the exchange rate at the time of the repayment of the loan, and because any such loss would be a capital loss.