IMHO there is no fixed rule in this situation. The meeting doesn�t need to have been already arranged. Compare these conversations:
A: I want to arrange a meeting before the end of the week. How about tomorrow?
B: I �m sorry I �m not going to be able to come tomorrow. Can it be on Thursday or Friday?
A: I want to arrange a meeting before the end of the week. How about tomorrow?
B: I �m sorry, I won �t be able to come tomorrow. Can it be on Thursday or Friday?