$300 is probably cheaper than a full month's rent I guess...
What about getting some other storage place, they drop it off at the storage place and pick it up later?
Give the board a call and see what they say:
http://www.ltb.gov.on.ca/en/index.html
You don't have any proof (direct or indirect) of the commitment by the landlord at all? I.e. a changed appointment time, or even speaking with the moving company about it?
If you have any supporting evidence you could either try to intimidate the landlord into honouring the agreement as it's a verbal agreement, saying you will sue him, involve the better business bureau, etc..
If not, you could try and negotiate it down, say $100 for the day and he avoids the court hassle.
Or just give up your security deposit and make it not worth his while and then accidentally get used oil stains all over the carpet

(I do not recommend this!)
It's a verbal contract, and the courts should favour you (they are always tenant friendly, at least in Alberta). Plus he's being an ass.