Contract on a Short Sale

In several post on the site I see a lot of questions about the “contract” with the seller(person being forclosed on). I’m new at the creative side of investing. I’ve attempted two short sales and could have been successful on the first but decided the equity was not quite enough. Someone please advise–In both attempts I collected an authorization to release and a notorized power of attorney from the seller. The contract then was between me and the mortgage company which seemed to work ok. In both cases they negotiated with me. Do you need a contract with the seller also or is this the correct procedure? Help!!

I use a listing agreement with the HO to secure my stake in the property.