My niece turned in her notice to her employer to accept another job. The employer asked her if she could continue to work some Saturdays and my niece

agreed. He told her to put the dates on the office calendar & she put 2 Saturdays from Aug thru Dec., 2013. He got someone else to work the Saturdays my niece wasn't working. The employer contacted her to turn in her key because the other person said she'd work every Sat. My niece has emails to and from the employer where he asked her to work what Saturdays she could - he'd give her first choice and then give the other person the remaining Saturdays. Is the employer legally bound to pay my niece for the 10 Saturdays she put on the calendar, which the employer had approved, or allow her to work those 10 Saturdays? If he doesn't, what would be her recourse?