This happened to us previously. I realtor friend had some clients come in on a Thursday and tell him that their home was going to sheriff's sale the following Wednesday. The realtor referred them to us and we got all of the short sale info in to the lender (Chase) on Friday. When we talked to the Chase on Monday, they stated that they would have to receive 50% of what they were owed before they would stop the sheriff's sale. That did not guarantee that they would accept the short sale, just that it would postpone the sheriff's sale.
The issue was that the homeowners were still going through a loan modification with Chase. The Loan Modification Dept even confirmed this to us when we called in. We advised the homeowners to speak with a lawyer to get it straightened out, but that we could do nothing for them.
Long story short, this is the reason to always speak with the lender a minimum of once a week, and preferably at least twice a week on every short sale you are negotiating. This type of thing can be stopped short when the sheriff's sale date is set. I realize that this would not solve your specific short sale, but this is directed more at short sale negotiations in general.