yes you can.
If you adjust the quote to reflect the NEW NET AMOUNT that you need to collect in the currency in which you originally charged, then when the terms are accepted (or you accept them) then the refund should automatically process. This does NOT refund the security deposit. You will still be able to refund that as you normally do.
Now, that said, if you are changing the currency mid-transaction, it might be better to refund 100% in the US dollars, and the switch and re-charge them in Canadian dollars? Otherwise the deposit refund will also be tricky.