Another way to look at it is the time spent already is a sunk cost. Figure out how much time it will take you to finish, and your effective hourly rate to get

Another way to look at it is the time spent already is a sunk cost. Figure out how much time it will take you to finish, and your effective hourly rate to get the remaining $10k. If it's a reasonable hourly rate, go ahead and finish. Otherwise, walk away. Also, that's why it's better to work hourly rather than fixed-price. That way you avoid arguments when the client increases the scope. You don't need to always have an airtight contract. What I do is make a limit on how much I'm willing to risk, and once that limit is reached I stop working until I get paid.