I don't think it's that tricky, you're on a US payroll in the US so the FICA is deducted so that bit is obvious. To avoid your employer having a Canadian payroll you register as self-employed and do the CPP in Canada yourself. The amount to withhold is based on the amount you've invoiced them, it's relatively straightforward.
This is explained in the CRA self-employment guide. You usually have to do installments in the second year, unless you're earning a tiny amount (I think the limit at the moment is $3,000).
Then you claim a foreign tax credit in Canada for whatever was withheld in the US, and when you claim your benefits when you're older, you claim under the provisions of the totalization agreement. How much you pay in tax at that point on your benefits will depend on where you happen to be living at that point, obviously.
-- Tue Sep 08, 2009 10:52 am --
agnelson wrote:He doesn't have to 'register' with CRA that he is self-employed: he is.
He needs to register to get a business number, GST number etc. so he can invoice them for the work done as a self-employed individual in Canada.
Steve.