Unfortunately cannot comment on the amount or the reason why they're expecting you to pay up without knowing more details of your tax situation, so your accountant would be the best person to guide. Technically speaking, you should only be liable for "difference" in taxes. The difference in taxes typically comes in due to differences in deductions, credits and/or tax tables/rates between the US and Canada. Canada, from what I know, typically has a higher tax burden than the US overall, so more often than not you'd end up owing "some more tax" to Canada. Please update us back once you get this resolved so we know what happened and why