Managed Payments will take your loonies, turn them into USD, and refund her.
However.
Currency exchange.
The American dollar is down about 3% today - which can change any moment. And that is the mid-market rate, not the "retail" rate we small change minions actually get.
So don't expect to know exactly how much will be refunded in loonies. It will be right enough but could change a few hours later.
Since the buyer seems to be a wheeler dealer (to be as polite as possible) it is very likely that if your refund is not to the penny what you agreed, she will continue with the NAD dispute.
You may find it worthwhile to go with sending the Return Shipping Label, getting back the (still saleable) item and then refunding the full amount.
Consider your options.