I would tell her in person, face to face. I would say, "I know you had mentioned that you may need to switch Mon for Tues, but I needed to let you know that I can't accomodate that switch as of right now. I picked up some extra business that ties up Tuesdays for me, and it is a regular set schedule so I went ahead and accepted it. I know that you were looking for that slot as a temporary schedule, but unfortunately I am not in the position to offer flexibility on that right now. What I have available right now is Mon, Wed, Fri (or whatever).

Reply