You should contact an attorney immediately. The problem you have is that you agreed to the sale. How long ago did you do so? If you are too late to rescind, you could be responsible for the commission, The buyer could also sue for specific performance which would mean that you would have to sell. In the alternative they could sue for damages actually sustained as a result of your breach. So, do not delay, call an attorney in the morning.
You need to seek an attorney's advice on this one. I don't think it is a good idea to take any position under these circumstances without getting advice first.
It does not sound like Duress is a good argument though if there was not some extreme pressure on you (usually physical pressure).