Can an arbitrator amend an award based on discussion with one of the parties without reopening the hearing?

I'd need more information to provide a specific answer, but as a general proposition, arbitrators are not supposed to have ex parte (private) conversations with one side (outside the presence of the other side). Depending on the particular arbitration forum (or court) in which this occurred, the rules might permit amendments of an award, but that'd likely be in very narrowly defined circumstances and for limited purposes. You might want to look into the possibility of seeking to vacate (overturn) the arbitrator's award, assuming that what you refer to as the "letter" actually was (or was made a part of) the final amended award. There could be a couple of grounds for doing so based on the information you've provided, but it'd be best to have local counsel look at the matter more in detail. Please note that I am not licensed to practice law in your state, and I practice only in NY. This response is provided for educational purposes only, and no attorney-client relationship has been formed. As they say, "Your Mileage May Vary." Best of luck.