I have it the other way around. I cash US funds into canadian accounts (have a US account too, but occasionally need it in Can$).
I just make sure to go to the teller and tell them SPECIFICALLY that its US funds. Even when I want it in my US account they screw up once in a while. Luckily the last time it was in my favor. She converted it with the exchange rate, but deposited it as US funds. So I got like 1.39 times the money I was depositing but it stayed US funds.
