No reason you couldnt have renegotiated on the spot....just say "I normally stick with the price I quote because I dont short my customers (hurts your rep you know)...but I guess I was mistaken somehow...what I thought we were biddibg on is different that what it here (tell him what you thought vs what is there specifically)...I really can olny pay $ x for it...I gotta eat you know!" . And then renegotiate it...you gave him facts as to why it was different...just be prepared to walk...dont buy a losing deal as that sets the precedent the next time he calls you. Negotiate fait and explain why...that will get you referalls and callbacks.