Q: If I am designating an employee as a key employee under the FMLA, can I just fire them on the first day of leave? 

A: No, the FMLA allows key employee designation (exempt, top 10% of earners), but it only allows denial for reinstatement not termination for beginning leave. One question to assess is: can this person be replaced in a timely fashion? If FMLA is four days, you might have trouble with that.