Hi Oliver,
If you did not sign a buyer broker contract, you have no obligation to continue working with that broker. Whatever the reason is that you do not want to continue working with that broker is something only known to you. You are free to use whoever you like as long as you do not have a contractual obligation.
Since this is a short sale, your offer is to the seller. The listing agent has a power of attorney to speak to the seller's lender on the seller's behalf to help negotiate the short sale process.
A commission is already built-in to the sale with compensation to the buyer's agent. You have two choices. Either be represented by the listing agent, or find another agent to represent you solely. If there is some reason the agent you were working with originally' created a reason that you would no longer want to work with them, then it would probably be a good idea to interview a few agents.
I don't see any advantage for you in going alone through this process. Because this is a short sale, the bank ultimately will decide how far in the red they are willing to go on this loan. You cannot go directly to the bank as you would need power of attorney from the seller to talk to their lender.
I just wrote an article on my website related to short sales please see the link below. If you need any help shoot me an e-mail and we can set up a time to meet for coffee. - Earlier today