וְדִבֶּר־ה֥וּא לְךָ֖ אֶל־הָעָ֑ם וְהָ֤יָה הוּא֙ יִֽהְיֶה־לְּךָ֣ לְפֶ֔ה וְאַתָּ֖ה תִּֽהְיֶה־לּ֥וֹ לֵֽאלֹהִֽים׃
And he shall be thy spokesman unto the people; and it shall come to pass, that he shall be to thee a mouth, and thou shalt be to him in God’s stead.
Rashi on Exodus
ודבר הוא לך means HE WILL SPEAK to the people ON THY ACCOUNT (לְךָ). This passage is proof, regarding every usage of לך and לי and לו and לכם and להם that follow after the verb דבר, that they all have the meaning of על “on account of”.
Sforno on Exodus
לאלוהים, so that he will perform miracles at your behest.