IMO the best way to deal with this situation is to do what you can to avoid it entirely, though that really comes with experience.
Here's how to avoid the situation:
- do your homework on the project.
Always discuss deliverables before starting work on something. If you get question marks over heads when the topic comes up, then you need to be prepared for the amount of work in front of you, and make plans and decisions accordingly. Its fairly common for an inexperienced video producer to be in need of audio education even though he's already halfway through his project, and its your job as an audio guy to be as honest and straightforward as possible with what you are wiling/able to do given the price and what his project will require in order to ever make any money.
Even experienced video producers and film makers can be sometimes shockingly uneducated with regards to the amount and type of sound work required to make their projects marketable. They don't all have to be works of perfection, but you'll know as well as anyone what can pass muster and what can't, and what kind of effort it will take to get from A to B.
If you can anticipate any compromises that will need to be made in order to deliver the project on budget and on time you'll be able to set expectations and minimized the compromise effect accordingly.
All of this comes from the courting process though, and the red flags that people throw up will become more apparent with experience.
I know, people hate signing contracts, but they also hate paying for audio work so you have to protect yourself. Contracts can be flexible by the way. You can specify a package rate for x deliverables and x hours, with an ability to continue work for a studio rate after those hours and deliverables are fulfilled. This way everyone knows what to expect, and it allows for a new entry point into the money conversation if new requirements appear as the project moves along.
contracts don't have to be all legalese and complex. In fact, the more straighforward and readable the better. Your goal with a contract is not to litigate it if your client wants other stuff - its to use it as a mutually agreed upon framework for the working relationship.
You don't need a signed contract for every job, but you should start considering it when the work looks like it will span more than a few days. Each situation is different so react accordingly.
also, a wise man once said that you should only have two prices - free and full pop. Anything in between devalues your work without diminishing expectations.
- communicate immediately when surprises occur
When you initially take assets - if you expect one thing and find another, then stop down for a moment and reassess. Then talk to your client before moving forward. If you see a problem and begin work anyway, then you will miss the opportunity to head off trouble before it can multiply.