I just went through the same thing a couple of days ago with a GMAC loan whereby THEY hired a 3rd party negotiator (REDC).  I received a call from REDC saying my commission was being cut from 6% to 5% and it was reflected as such on Equator.  I rejected the banks offer on Equator and instead countered back on Equator at a 6% commission.  (This is not a Fannie/Freddie loan so my commisison isn't protected at 6%).  Anyhow, after my counter offer in Equator was submitted, I received a phone call from the negotiator saying GMAC only pays 5% when two agents are involved.  I told him his payment should come from GMAC as they hired him.  He said basically, "take it or the bank can foreclose".  I hung up the phone and spent the next hour calling GMAC customer service where I spoke to two managers.  I specifically asked NOT to be connected with the short sale dept.  I voiced my opinion to these 2 customer service guys that Bank of America and Chase didn't cut my commission, and I would remember GMAC when this market turned around.  I also asked them to pass the message on that I would have to think twice about helping another client with a GMAC short sale.  I was told both customer service managers would pass my complaint to "higher up".

The next day I received an email from the REDC negotiator stating my commission was 6% (5% to me and 1% to REDC.  What a joike! I told him if my commision was 6%, that is what I would get.  I then told him I was not going to hold up the sale for 1%, but that I was going to spend the rest of the day googling and calling GMAC board members to complain.

An hour later I had a call from the negotiator stating GMAC had made an exception in my case and my commission was being taken back to 6%. 

Good Luck to you!