Normally when I get a security deposit from someone it's in combination with signing an agreement and giving keys (and often first month's rent). In that scenario the tenant has clearly taken possession and if they decide to not move in then the rules around that kick in (on a month by month they have to give 30 days notice, the security deposit can go towards unpaid rent, on a 1 year fixed term they're actually responsible until the unit is re-rented up to 1 year).
As said it's a verbal deal since money changed hands, but if the tenant doesn't have keys then I'm not sure if they are considered to have possession or not.
From the landlord's point of view, once they have a security deposit it's rented, so usually they have taken the unit off the market, stopped advertising, told other prospective tenants that it's not available, etc.
If this was for November 1 then the security deposit covers November's rent. There's still time, if it were me I would ask the landlord to refund me if they rent it for November 1, or refund me a partial amount if they rent it sometime in November. If the landlord is keeping the security deposit and it's been re-rented right away (i.e. there's no actual loss on the landlord's part) that probably wouldn't look good.
Service Alberta has a phone number they'll give input on the laws (they're tenant slanted):
http://www.servicealberta.ca/contact.cfm
There's also a mediation process which will mediate and make a binding decision that's relatively inexpensive.
In the future giving money to a landlord (or anyone really) where there's some expectation of being able to get it back without something in writing about what the parameters are for getting it back isn't a good idea.