Or HaChaim on Genesis 32:18/אור החיים על בראשית ל״ב:י״ח

1 ויצו את הראשון לאמור, He instructed the first one (messenger) to say, etc. The apparently superfluous word לאמו was that even if Esau were not to phrase his enquiry in exactly the words Jacob assumed he would, he would no doubt use words to that effect.

2Jacob may also have used the word לאמור to tell his messengers to use exactly this formula even if they would not be asked directly by Esau but by some intermediary. He added the word ושאלך, "and if he will ask you," to indicate that such a question might be addressed to the messenger second hand, not by Esau himself.