Or HaChaim on Genesis 43:2/אור החיים על בראשית מ״ג:ב׳

1 כאשר כלו לאכול, when they had finished eating, etc. Perhaps the Torah mentions this fact to indicate that they had just enough grain left to last them for a journey to and from Egypt. The reason the Torah describes the supply as if it had already come to an end is that now there was a need to bring more grain. It could also be that the Torah considered the food consumed by the brothers while journeying back and forth as part of the supply that Jacob had stored in anticipation of the famine as we have learned in Taanit 10 based on the words למה תתראו in 42,1. This is the reason why the Torah emphasises that what had been eaten up was the "grain they had brought from Egypt" as distinct from their total store.