This is an idiom. The Israelites considered the new day as starting when the sun went down. Alternate translation: "you should pay the man on the same day he does the work" (See: [[rc://en/ta/man/translate/figs-idiom]])
The full meaning of this statement can be made explicit. Alternate translation: "because he is poor and depends on his wages to buy his food for the next day" (See: [[rc://en/ta/man/translate/figs-explicit]])