Well... I hope you have a seasoned agent representing you. This is the first of MANY questions you'll have. In reality, a Bank Approved Short Sale is somewhat misleading. The banks approve specific offers and terms. If that offer falls apart and is quickly replaced with a like contract, it has a very good chance of being approved in a shortened time frame. If it has been awhile, or the offer terms are significantly different, the time period is lengthened.
Get a copy of the approval from the listing agent so you know what terms you need to meet. Then, don't mess around - if you're agreeable to those terms, write your offer exactly the same or as close as you possibly can. Get it ratified and sent to the bank quickly.