Dr. Robert Mayo was a tenured engineering professor at North Carolina State University (NCSU). In July, he informed his department chair, Dr. Paul Turinsky, that he was leaving NCSU effective September 1. Turinsky accepted the resignation. In October, after Mayo had departed, Phyllis Jennette, the university's payroll coordinator, informed Mayo that he had been overpaid. She explained that for employees who worked 9 months but were paid over 12 months, the salary checks for July and August were in fact prepayments for the period beginning that September. Because Mayo had not worked after September 1, the checks for July and August were overpayment. When he refused to refund the money, NCSU sued. The first step was a hearing before an administrative agency. At the hearing, Turinsky and Brian Simet, the university's payroll director, explained that the "prepayment" rule was a basic part of every employee's contract. However, both acknowledged that the prepayment rule was not included in any of the documents that formed Mayo's contract, including his appointment letter, annual salary letter, and policies adopted by the university's trustees. The university officials used other evidence, outside the written documents, to establish the prepayment policy. Based on the additional evidence, the agency ruled that NCSU was entitled to its money. However, Mayo appealed to court, and the trial judge declared that he owed nothing, ruling that the university was not permitted to rely on parol evidence to establish its policy. NCSU appealed.

Can NCSU rely on oral evidence to enforce its advance rules? why?