You have a contract with the university in which the university provides classes for which you pay tuition.If there is a mistake in a contract by one party, and the other party did not know and had no reason to know of the mistake, the contract is enforced on the stated terms against the mistaken party.Therefore, if you did not know and had no reason to know of the billing error, the contract is enforced on the mistaken terms against the university and you are not liable for payment of the additional amount the university is claiming to correct its billing error.