When you wrote the offer your agent would have included a short sale addendum with a time limit for the bank to approve the sale. If that time has come and gone and the bank has not yet approved the sales price and terms in your offer then you can back out. Additionally, if you still have contingencies for inspections, disclosures, loan, appraisal and other caveats that have not been removed by you in writing then you can back out via one or more of those contingencies.
讀讀contract. :)