Eliezer’s Refusal to Eat Before Completing His Mission

לא אוכל עד אשר דברתי דברי ]כ"ד ל"ג[
As Eliezer was sent as a messenger from Yitzchok to be Mikadish [Kedushin] Rivka. And a Choson on the day of the Keddushin is forbidden to eat until after the Keddushin. Therefore, Eliezer said, “I cannot eat until I finish my words - the Keddushin."

Sefer Kedushas Levi

We can also say that Eliezer wanted the forthcoming meal to be a Seudas Mitzvah. Hence, he said, " I cannot eat until I speak my words to complete the action of Kiddushin, and then it will be a Seudas Mitzvoh."

Sefer Kedushas Levi