on call dispute with for profit hospital.

I am a supervisor in a for profit hospital. I feel there are many issues which I have been given an "unfair shake" here, promotion practices included. I was recently given extra pay for carrying a pager. I was under the understanding it was for the hours I would be carrying it. It was approved for payment during non working hours. I do payroll, therefore, I put the hours I was on beeper on payroll, including the days, and had this signed by my boss to show it was approved. Now they are telling me only part was approved, the rest I was expected to be on call for free, and I have to pay back part of it which is approximately $1200.00. I feel the payroll was signed, and they can't take it back. Do I have a valid arguement?