I assume you're referring to item # 221363901641, the toy telephone? I'm also assuming that the buyer did not include her stipulation about paying later as a condition of her offer -- if she did, then you may be stuck with it if you've accepted her offer with the attached condition.
However, if that request (to pay later) came after your acceptance, then you have options:
1) Even though you may be desperate for cash, it's not an enormous amount (around $100?), and you may make as much in additional sales soon anyway. So, if you want to remain on good terms with this buyer, send a message to her thanking her for her offer and saying that you will agree to the delay in payment as a one-time courtesy, but that you are unfortunately not able to agree to a delay beyond the close of business on March 2nd. Don't say why, and no need to explain further.
Make it sound businesslike, and don't say anything about your personal situation and needing money. Check on March 3rd, and if payment in full hasn't been made, you can open an Unpaid Item Case (UIC). In my view an experienced buyer like this probably knew better than to make an offer if she couldn't pay right away, and she may have been counting on your inexperience.
On the other hand, she may really have a legitimate issue with paying right away (some other urgent expense arose in the meantime perhaps?). By giving her what she's asked, you may make a very happy customer indeed. There is just no way of knowing what the buyer's true motives are, and in the end, it's irrelevant. Even if you find her request "fishy", you can play along for the sake of concluding the sale and making a satisfied customer -- but set your limits and don't go beyond them.
2) You can send the buyer a message indicating that unfortunately you're not able to agree to her delayed payment, and ask that she please pay within, say, 24 hours. Keep the message polite and simple. After that point, once you're beyond 4 days post-purchase, you can open a UIC against the buyer.
However, keep in mind that:
(a) eBay will give your buyer some time to pay, and your item will be in limbo until the case is resolved (i.e. you won't be able to relist it); and
(b) if the buyer does finally pay, she'll be able to leave Feedback and DSRs, which could potentially seriously harm your fledgling business if she decides to retaliate because she feels you were unfair.
In my view, for future transactions, if you need funds so quickly, I recommend you require immediate payment on the more valuable items. This option is available during the listing process, or you can revise your items to include it. Also, I haven't looked carefully at your listings, but you should make sure, for all other items, that you indicate in the listings when you expect buyers to pay - 3 or 4 days is probably reasonable.
If you're not selling on eBay with a business plan that anticipates some losses, but rather as a means of getting badly needed quick cash, you are going to have to be very careful. The eBay of today expects all of us sellers to operate as professionals, as if we were running online businesses.
You are aware of Paypal's 21-day holdback of funds, but there are also various selling limits that eBay itself places on new sellers. In effect, you are still on probation as far as eBay is concerned. As a new seller, you can't afford negative FB, poor DSRs and other demerits without getting penalized and perhaps restricted from selling (or even removed from the site by eBay). You have to decide how important it is for you to be able to keep selling on eBay.
I don't usually like to give a definitive opinion, but if I were in your precise situation, as a new seller, desperate for cash, with a FB score of only 2, I would agree to this buyer's request as set out in point (1) above, and bend over backwards to be friendly and businesslike until March 3rd. I say this even though I think she is probably taking advantage of your inexperience. After March 3rd, you'll have to take steps to be paid (open a UIC).
Next time though, decide which items you can't afford to not get paid for right away, and set them up with "Immediate Payment Required".
Incidentally, I don't understand your comment about losing $20 on currency exchange. If you're listing in $US, you will actually gain anywhere from 7% to 9% at the moment on the exchange into Canadian. Paypal's fees for converting currency are still pretty low, certainly not enough to eat up the difference between $US and $Cdn value.
