Mutual acceptance begins when the buyers and sellers have accepted the deal in writing. Just make sure to include "subject to short sale approval" in your offer somewhere. This way the contract is contingent upon the approval from the short sale.
Most banks will not entertain an offer that is sent in without execution from all necessary parties. They don't want to waste their time with something if they don't have a solid deal.
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Len McGuirk
Prudential Americana Group
Direct: (702) 203-6688
Las Vegas Real Estate