So is this a partnership, or a corporation?  And is it a dispute between two shareholders of a corporation?  Are you sure that both of them agreed to your being hired for this work?  And who is your client, anyway?  Seems to be like it's the arbitrator, so you need to find out if he is going to pay you or he is just assuming that the corporation will.  

Many arbitrations involve a hearing, and you may be called as a witness.  Do you know when and where the hearing would be held?  Does your agreement provide for payment of preparation, travel and appearance time?