What is the ruling on resorting to a usurious Canadian bank to finance the purchase of a house when an Islamic financing option is not available, and does the sin increase if I intend to pay the full price as soon as an Islamic alternative becomes available?

If dealing with the bank is permissible, there is no objection to it for your need and to avoid hardship. However, if it is forbidden, such as a usurious loan, then it is not permissible except in cases of extreme necessity where there is a fear of perishing. The need to own a home or luxuries is not considered a necessity that permits usury. Borrowing with usury is among the definitively forbidden acts, and it is not permissible except out of necessity. Necessity is estimated by its extent, and this does not include owning a home as long as rental housing is available. If you have become involved in a usurious transaction, you must repent, and it is preferable to expedite repayment in exchange for dropping the interest, if possible.