You should always bargain the graduation time with your academic mentor instead of with the industry supervisor, i guess. A good version is that the graduation would be the next Janurary, while you could start working as early as possible (this is tricky). When you are sure about the offer, talk it with your mentor or ask for a sponsership from the industry supervisor. You are always on a winning side if you hold the offer in your hand. Either your boss will let you go, or she owe you one and you can bargain something else later.