I had a bug after clearing out Redoran's Retreat as Farkas' first job for me in the Companions. After killing everyone inside RR (and screwing around with the sgtm command, possible cause?) the quest showed up as completed even though I hadn't returned to Farkas. The map marker lead me to the Companions base (no way I'm spelling that out) but after entering the map marker pointed into a far away place, but not OUT of the base. Teleporting Farkas to me didn't help either BUT I was able to receive a different mission. Farkas tasked me with killing a bunch of Falmer (nice quest for my level, gg balance) and after completing that quest and returning to him again, BOTH quests are finally fully completed. I have no idea what caused this bug in detail or if my "fix" works for everyone, however trying it out is always possible.
To teleport Farkas to you (works only on PC to my knowledge) you open up the console via ~, type "prid 0001A693" without the " and then "moveto player". This should have Farkas appear right beside you.