Or HaChaim on Genesis 29:20/אור החיים על בראשית כ״ט:כ׳

1 ויעבד יעקב ברחל, Jacob served for Rachel, etc. The reason the Torah mentions ברחל is that Jacob made a public announcement at the time that his service with Laban was for Rachel and that her hand in marriage was the wages Laban had agreed to pay him in return for his service. A major reason he made this public pronouncement was to make it clear that he did not serve for Leah.