Is this verbal contract binding

I was offered $7K for going back to night shift. I received half up front and have been working for over a year. I am not supposed to receive the other half until mid-Feb, 2007. Verbally I agreed to 18months, however, I am tired of the often dangerous situations with the patient loads,etc..and am actively looking for another job. If I didn’t ask for the rest of the money, should I be required to pay back the first half if I quit before the 18months are up.