Legal terminology question

[BitVyper]

So I'm a little pissed off at my property manager, and I want to track down some legal information before I talk to her boss. Unfortunately, I think it's something covered more in contract law than tenant law, and I'm having some trouble looking for the right terms, so I'm hoping you guys can help me narrow my search a bit.

Without getting into the details too heavily; after being out on-site for most of the day, my property manager decided to close the office before their posted hours on the first of the month - I actually have their admission on this fact. When I DID pay my rent, they tried to tack on a late fee, which I gave them hell for, and they removed. Now I've got a notice saying I owe them a late fee (late fees ARE in my rental agreement). So basically, they trapped me into this. The late fee itself is a petty little thing, but I don't like getting dicked around, and I want to make that absolutely clear to them.

So I'm hoping someone around here *cough*Snake*cough* knows the right terminology for this sort of thing, so I can track down the appropriate section of Alberta law, just incase my property manager's boss decides to be a dick too. Obviously, if it comes right down to it, I'm probably stuck for this month, but that won't stop me from using the knowledge gained to ruin their shit in the future.

Anyway, if any of you happen to have an idea of what I should be looking for, I'd love to hear it.