If paying your student loans was contingent on you making $60k, you made $55k, and they came knocking at your door demanding money yes I do think you have grounds to sue and not pay them.

I don't care one iota about the legal minutiae that says they're technically allowed to collect, it is blatantly obvious what the students thought they were signing and so it's really hard to argue "no actually they agreed to <this other thing>." If you present paperwork that you say is the legalese version of the agreement you discussed, have the other party sign it on that basis, and then it isn't that is the lowest hanging fruit of contracts that should be thrown out.