ויישם [וַיּוּשַׂ֤ם] לְפָנָיו֙ לֶאֱכֹ֔ל וַיֹּ֙אמֶר֙ לֹ֣א אֹכַ֔ל עַ֥ד אִם־דִּבַּ֖רְתִּי דְּבָרָ֑י וַיֹּ֖אמֶר דַּבֵּֽר׃
Y pusiéronle delante qué comer; mas él dijo: No comeré hasta que haya dicho mi mensaje. Y él le dijo: Habla.
Rashi
עד אם דברתי UNTIL THAT I HAVE SPOKEN — This is the same as עד אשר, so that you see that אם has the meaning of both אשר and כי, an example of the latter usage being (49:10) “Until that (עד כי) men come to Shiloh” (where עד כי is equivalent to עד אם in our verse). That is what our Sages, of blessed memory, have said (Rosh Hashanah 3a): The word כי is used in four meanings. One of these they say, is אי which is the Aramaic for the Hebrew word "im".
Alshekh
I will not eat. He was afraid they would claim that the gifts he gave Rivkah were merely payment for his meal rather than engagement presents.
Alshekh
Alternatively, Eliezer perceived the angel that had come to kill Besueil (see Rashi v. 55) and wanted to conclude the betrothal before he died, lest it be delayed on account of the mourning.