Sforno on Exodus 6:28/ספורנו על שמות ו׳:כ״ח

1ויהי ביום דבר ה' אל משה, when G’d spoke to Moses that he should speak to Pharaoh and he answered G’d that Pharaoh would not listen to him, i.e. after G’d had already appointed Moses and Aaron as the leading authorities over Israel as well as over Pharaoh, as reported, G’d explained that He had not intended to give both Moses and Aaron equal status, but that Moses would be an authority, G’d like, for Pharaoh, whereas Aaron would be his mouthpiece, his interpreter.