This was an out right con game played by this HO, in no way can I see this as anything other then that, and it was the whole reason the HO changed the original contract in the first place, probably hoping that the EC would not catch it.
A contract is written to protect the EC, from the non payment of the person who is responsible for paying the bill, if you have the original contract he e-mailed the HO, I would just play dumb, and say hey he change it after the fact, but I don't even think it matters, as any court of law should sway enough your way on the preponderance of evidence that you would win anyways, as was said, this is just outright fraud.
I would say you entered into a good faith contract to do just what you said, and this should stand up in a court.
Your time and money that has been invested in the job already plus the court cost and attorney fees, is yours, if for nothing else, I would slap this guy in court, just to show he can't be playing games like this.
If someone was to try something like this with me, I would ask him if he thought I was drunk, because if he didn't I would say he was, thinking of trying a trick like this.
I will go after a con in a heart beat, if for no other reason, but to maybe keep him from doing things like this to others.
Any court should see this clear cut, of someone who is trying to take a free ride on someone Else's dime.
Anyone who can't see this has to be blind.
and don't forget to add in a fair interest rate compounded monthly to the bill that goes over the expected 30 days net.
Get a lawyer and get one fast, don't wait. be the first to act. I would simply tell this HO that there is no way I'm am going to rewire your house for free. or even at a lower rate then I would normally charge, send him a certified letter, along with the bill and the original contract, and let him know you expect payment for the work completed. but do this after talking to an attorney. also I don't know about there, but around here, it is better to get an attorney from somewhere away from the area. he might even be able to force the inspector to sign off on the work your did, and land the extra work on the HO, this would certainly make the contract fulfilled.
But remember, our suggestions are only opinions of what we would or would not do, and are by no means, legally usable in a court of law.
What ever choice you make let this be a learning lesson, to never let the receiver of a contract to ever change a contract with out it voiding the contract, and especially one that has potential of costing you more time and materials for the same price. if they want it changed then say ok I'll rebid it, after you do the research of what it might cost you for the "what if" and then put it in the contract.
Can ya tell I can't stand a con?
