Here is a good example of this happening. A real estate agent had a short sale with one of the largest lenders in America. The short sale negotiator said that they would not approve the short sale unless the seller came to closing with 50k cash. The sellers said no way.
Fortunately, the agent knew that Fannie Mae owned their loan. The good news for the seller is that Fannie Mae does not ask for promissory notes or pursue deficiencies. (There are a couple exceptions, but they didn't apply here.) If those are Fannie Mae's Guidelines, then why would the short sale negotiator require the seller to bring 50k to closing?
Turns out the negotiator was not following Fannie Mae's Short Sale Guidelines. The agent contacted Fannie Mae directly. Within 72 hours a supervisor called from the short sale department. This time they were singing a different tune.
The sale was approved and the promissory note waived. The seller got to walk away owing zero. Why do things like these happen? Because, in our opinion, there is little oversight over the short sale process. Nothing happens when a short sale gets rejected and sells for less as a bank owned property. No, don't kid yourself, this happens a lot.
These short sale negotiators develop an adversarial mindset. They probably even get paid bonuses based on how much money they squeeze out of short sale sellers. This causes their ego to get in the way of what is best for the owner of the loan. If you are thinking about short selling your home, make sure your agent knows how to research who the owner of your loan is. That info will allow them to negotiate from a position of power. And it will help you avoid a deficiency or promissory note.