Also foreign institution here! From what I understand, offering you the CGS-D just means you got ranked near the top, but practically you would only end up with the PGS-D in terms of money, since the CGS-D is restricted to Canadian institutions only. I'm pretty certain you can't keep the extra money, but I have seen people add the extra line in their CV about declining the CGS-D in favour of the PGS-D. A friend of mine in a different committee also got the CGS-D this year, even though they were also at a foreign institution.
Adding on to your second paragraph, do you know where and when we would check in June to see our feedback?