A & B entered into a binding written contract whereby A promises to hire B as the manager for his health food store for a period of 24 months and B promised to work in store 8 hours a day, 5 days a week for $800 a week. After two months, B comes to A and asks for a $50 a week raise. A orally agrees, provided B promises to work an extra hour on Friday nights. B works 9 hour on Friday for the next month. A refuses to pay B the extra $50 and seeks your advice on his legal rights.

Advise him, making sure to answer these questions:

A) Would evidence of this oral agreement be admissible or in violation of the "parol evidence rule"?

B) Is there a valid legal modification of the contract between A and B?

C) Is the promise of A to pay an additional $50 per week legally enforceable? Explain fully.