Polinia the priest told Mires that the mercenaries would not receive their reward until they retrieved the grain sacks from the Kerub dens in Akarios Plains.
† All quests reward XP but NCSoft is fond of changing the amounts frequently, to the point that it is simply not wise to try to track the exact amount in a wiki.
Quest Notes
Kerub Grain Sack
The sacks are found within the small fencing in the Kerub village, a couple in each fenced area.