How to remove Apple ID balance to use a Canadian gift card

If your Apple Account is registered to the USA at this time, only the US Apple Support can clear the balance for you.

The other option would be disabling the Activation Lock, erase the iPhone and create a new Apple Account for the country you are planing to use this device and content from Apple. But you will not be able to use your previous purchases after that.

How to create a new Apple Account - Apple Support

Only the official Apple Support can do that, use the blue link below as a first step:

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• If you have less store credit than the cost of a single item, contact Apple Support.

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copied from: Change your Apple Account country or region - Apple Support

To which country is your Apple Account registered to?