What can I do if the terms of my adjunct contract are not what I was led to believe they would be? The situation is as follows:

Once upon a time, the university where I have served as an adjunct decided to bring me on in a fixed-term role spanning a single academic year. This was to get them out of a low-manpower bind due to medical and retirement cases with some of our tenured faculty. The idea was a reasonable salary with benefits. So, classes start, and I have no contract. One pay period goes by, then two. Finally my contract comes through after much back-and-forth between the administration--which I'm to understand is heavily associated with our business school--and our department head. Result? Reduced from a single year to a single semester to avoid the benefits package, and reduced pay because they say they cannot retroactively pay someone who didn't have a contract during the intervening time. Given my history with this institution, I'm not at all surprised by this, but this is something of a new low. Yes, it makes good business sense to acquire labor for as cheaply as possible, but it also sends a bad long-term message that I am not valued as an employee. What, if anything, can I do about this?
First of all, consult a lawyer before doing anything.

If you were to leave in the middle of the course, that will cause havoc, that they won't probably want. Since they are not paying you for the work, you are under no contractual obligation to fulfil anything (but consult the lawyer), and can leave without giving them notice.

Now, you may find unethical to leave your students hanging like this, but the mere threat that you are willing to do it, may get some wheels spinning to give you a decent offer.

Another option to consider is whatever anti-slavery laws apply in your jurisdiction, since you are working for free; and anti fraud, because you are working without a contract. Furthermore, since you don't have a contract, you are not covered by the insurance, and if anything were to happen during your lecture, there is trouble. Think who would have to pay for in case, for example, a projector caught fire and ruined the classroom.

If your oral agreement is considered a contract, you can't do this; but then they would be acknowledging they have to abide by these rules.