Except it wouldn’t be about poor negotiating. In contract law this is referred to as a Mutual Mistake.
Taken from the Cornell Law School website:
In contract law, a defense used by one party to argue that a contract is invalid. In order to use the defense of mutual material mistake to argue that formation of the contract was improper, a party must argue that: (1) there was a mistake; (2) that the mistake must be material, meaning, that it must concern substantive characteristics of the subject of the contract; and (3) the mistake was mutual, meaning both parties had the same mistaken belief.
The evidence for this? The new MOU
An employer has a duty to pay its employees correctly 100% of the time. If at any time the employer becomes aware of an employee being compensated for less than what they are due, then said employer has the responsibility to pay the employee what is owed.