The reason: If the property's being offered as a short sale, that means it's likely that it's a preforeclosure--that the property is heading down the road to foreclosure.
Your scenario seems to be that you'd put in a contract to purchase the home in October, and rent it in the meantime. I doubt that a lender would approve a short sale with a proposed closing date in October. It might be worth a try, but I seriously doubt it.
Next, remember that foreclosures and short sales are handled by two different departments in a bank. They're parallel, but different. And it's always uncertain which will happen first. I've seen plenty of cases where someone's put in a contract for a short sale and the bank's approved it. Still, it gets sold as a foreclosure and the would-be short sale buyer loses out. The same thing could happen to you: Even if you got your short sale offer accepted, the bank could foreclose before October. And if that happened, your contract to buy would be worthless.
Next, of course, are you SURE you'll qualify for a home loan in October? How are you so sure? What if, for example, you lose your job? What if interest rates jump up a couple of points? Even if the bank agreed to a short sale to occur in October, and even if it didn't foreclose in the meantime, what would you do if October rolled around and somehow you didn't qualify?
I'm not a lawyer, but what you're proposing is legal. It's just not a good way to approach it. Either do a conventional lease-option (or in your area land contracts are much more common)--and make the term for at least 2 years--or else wait until your credit is good enough for you to buy conventionally.
Hope that helps.