NOT A PSAK, BUT A SLIGHTLY INFORMED OPINION:
It really depends on what the pushka is for, and how great your need is for the cash. If the pushka is a “general” tzedaka box – i.e., you put money in periodically, and when it’s full you decide who to give the money to – the money there is generally designated for aniyim (assuming you haven’t generally designated it for institutions or yeshivos). At the moment you are strapped for cash – i.e., you need some cash, the bank is closed, you can’t get to an ATM before you need the cash, ect. – you have what to rely on in saying that at that moment you are an ani, and may take tzedaka for yourself, all the more so because you intend to pay it back shortly. Of course, this would depend on how great your need for the cash really is – but that is something you can best judge for yourself. Bottom line, if you really do need the cash right away, you would have what to rely on to consider yourself an ani and take it, especially as only a very short term loan.
BUT, if the tzedaka box in question is designated for a specific cause – i.e., a yeshiva or some other organization sent you a pushka, or you mentally designated your home pushka to go to a particular cause – likely that cause gains rights to that money as soon as you give it. They could, in theory, sue you in beis din if you failed to give the money to them and instead gave it to another tzedaka, and therefore, it would be hard to find a basis for you to use that designated money, even if you could be considered an ani due to your present need for quick cash. One solution might be to write a check for whatever you take and place the signed check in the pushka – the signed check represents your obligation to pay no less than does the dollar bill in the box, and might solve the problem.
Interesting question. Other thoughts: What do you do if you do borrow the money from the pushka and the value of the money appreciates or depreciates before you replace it? What if you realize some sort of profits from the borrowed money – i.e., you use it to buy a lottery ticket and win – was the borrowed money yours because at the time you took it you were an ani, or was it the tzedaka’s money that was lent to you? Lots of possibilities.