Reb Shlomo Kluger explains the verse based upon our Gemora (Daf Yomi: Gittin 29a) . Rava had said that our Mishna was referring to words (that were said by the husband, i.e. instructions to write the get), and those cannot be passed on to another messenger (for words are too insignificant to be transferred). However, something tangible (like a get) can be passed on to another messenger.
This principal, he explains, is that mere words cannot be transferable to a second agent. However, the first agent can be appointed for mere words.
Moshe’s mission was one of words. Hashem commanded him to go to Pharaoh and speak to him. Accordingly, one can ask: How did Moshe have the authority to transfer this to Aaron? Mere words are non-transferable to another agent!?
This is the explanation for the introductory verse. Hashem made Moshe into a lord over Pharaoh. Moshe, therefore, was not merely an agent; he was the principal himself. He, therefore, had the authority to appoint Aaron to be his agent to talk to Pharaoh.