Fluctuating workweek

Q. My work hours fluctuate a lot depending upon the flow of work. To help manage its cash flow, my employer asked me to sign an agreement where I would receive a set salary each workweek regardless of the number of hours I work. In weeks where I work more than 40 hours, I am paid half-time for overtime hours, instead of time and a half. Can my employer do this? My position is nonexempt ...