A Facebook post by Will Duffy, founder of the Collaborators Project on Facebook group God is Open:

The burning bush story is a great picture of who God is and how He relates to man. He spends a great deal of time telling Moses what he will do and what he will say. But Moses is fearful and essentially rejects God’s plan. So God gets angry and says, “Fine! Have your brother Aaron do it.”

What God said Moses would do, Aaron ended up doing. What God said Moses would say, Aaron ended up saying. Prophecy is not pre-written history.

Ex. 4
12 Now therefore, go, and I will be with your mouth and teach you what you shall say.”
13 But he said, “O my Lord, please send by the hand of whomever else You may send.”
14 So the anger of the Lord was kindled against Moses, and He said: “Is not Aaron the Levite your brother? I know that he can speak well.