When writing a offer for a short sale the SSA or Short Sale Addendum form is included. This form states rather you give a deposit with the offer or wait for lender approval. It also states how long you are willing to give the lender to respond. It also states when the inspections and contigency periods commence. Make sure you review and understand this form when making an offer
As long as you are not under Contract, (Contract has been signed by both parties) you may certainly walk away. You would simply submit CC, Cancellation of Contract. Since there is no Escrow, your Deposit Check should not have been cashed, you may simply ask for it's return.
I would hope that you have a Realtor representing you and wonder why you are asking us these questions?
Good luck and may God bless