If the Agent wrote the offer, submitted it and is the procuring cause
then you have to pay. Sadly, if it is now a Short Sale, the banks are
covering the Short, so it will likely be negotiated down.
Clearly, it seems it is a regular sale now gone short sale.
Believe it or not there is a lot of work on the part of the listing agent to close a short
sale, even though it is not apparent to you.