There are several bugs involving Hendak's disappearance or even turning hostile.
You did nothing wrong and therefore you should just 'cheat' him back in using
CLUAConsole:CreateCreature("Hendak")
You have to have the cheats enabled to do this (to do so, make a backup copy of the baldur.ini file, and under the header Program Options, enter the line:
debug mode = 1)
This will create a new Hendak to whom you can talk after finishing the slave quests, and he will give you your reward.
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Melusine, High Queen of Fluffies, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia