I've seen this happen to "others" before and you have to fight nasty with nasty. While on the project did you notice any other projects or improvements including the ones you did that would require a building permit that wasn't taken? Have they had a visit by the local building inspector lately? Have they had a vist by the local assessor lately so that all improvements can be added to their structure evaluation so they are paying the fair property taxes on it? Were any of the materials or labor paid for by their personal small business but actaully installed in the home? If so the tax folks will want to have a chat about that. Does the sprinkler system have a permit, was it approved, does it have proper anti-backflow (none of 'em do, very spendy)? Health dept. will want to know about that one. Do they have a tax re-sale number that they are using as a false business to buy the materials tax free? Do all bedrooms have coded ingress/egress sized windows, most folks call it a den then add the bed later. Inspector will be all over that one.
I could go on and on here but I don't want to sound like a cruel bastage who is willing to ruin the lives of someone who tried to short meon something as material as cash. I'm not like "others" lol. You get my drift here, compile a list of potential issues that will weigh up in $$$$ and headache way faster than just cutting you a check if you decide to shine light on them.
There are a ton of others I don't feel are forum appropriate, if you dont get resolution, pm me, I'll hook ya up with some good stuff.
Sorry your going through this, the days of verbal are over, better get a basic contract template from now on, and just in case, have your eyes open while on the job in the event you need ammo later like the "others" have had to. Have a good one. EW