There's no "proper way" to ask for the sale price. You can ask them, and if they're motivated to sell it, they might agree.
Some suggestions on how you might approach it:
"I was in here a few weeks ago and saw it on sale for $1500. Is it likely to go on sale again any time soon? $1500 is my upper limit."
This is a pretty passive way to go about it. You're not actually asking them to discount it, but you're setting your price expectation. They might agree to $1500, they might be able to tell you when it is next discounted (leaving you to decide if it's worth the risk of waiting that long) or they might not be able to offer you anything.
"I was in here a few weeks ago and saw it on sale for $1500. Even though I missed the sale, is there any chance you could sell it for $1500?"
More direct, but still polite. They'll either say yes or no, or they might give you a counter offer.
"I'm willing to offer $1500 cash for this [item] today. What can you do for me?"
Sometimes a guaranteed sale is motivation enough. This is probably more for a situation where the item is more commonly financed (ie, store card, payment plan, etc.)
"$1700 seems a bit too pricey, what's the best price you can do for me?"
This is open-ended, and they might even come in under what you're willing to offer. They might also refuse and tell you $1700 is the asking price, so that's what you pay.
Ultimately, it's up to you how you approach it. Given that you are willing to pay the asking price and that it's difficult to source elsewhere, you can ask for a lower price, and if the seller doesn't budge, then you can just buy it. Some negotiation tactics require you to be willing to walk away - in your case, I wouldn't suggest it. Be as polite and charming as you can, ask for a discount, but don't stress if it doesn't happen for you.
Note: I can't offer any advice culturally - I don't have any particular insights into Canadian or Chinese-Canadian culture. :)