Not bashing on you, Hoofpic, but this is exactly why I don't allow anyone to "work" in my barn when I have boarders. Either you are a boarder or you are hired help in my barn. Hired help gets a pay check and then they pay board just like any other boarder if they keep a horse here, or they're a boarder and they come visit their horse and enjoy it and they go home. A boarder will NEVER be allowed to clean stalls or feed or any other duties for a reduction in board money. It just gets too messy. And if it's gotten messy, that's the BO's fault for allowing it to happen.
The correct thing to do is to pay for Sept. board, unless it specifically states in your contract that they will pro rate a partial month, stop doing the feeding and move your horse during the 30 days. Don't expect a reduction, don't expect any favors. Take it as a learning experience and move on.