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How should I record invoices in foreign currency in GNUCash?
It depends upon in how many currencies your business is denominated. If your business is solely dependent upon this one payer, it's best to start up a new set of books in USD.  All accounts should be translated from CAD from a date preceding the USD activity.  The CAD books should be closed, and all should be done with the new USD books. If your business will continue to use both USD & CAD, it's best to have two sets of books, one for USD and one for CAD.   Multi-currency books are a nightmare and should be avoided at all costs. Also, with the way you describe your situation, it appears as if you're also blending your household and business books.  This should also be avoided for best practices.