You'll find the slave outside in the plantation, though if the guards see you free him they will attack you (you may want to kill the guards first to keep the slave safe, or try using a [[Morrowind:Command Humanoid|Command Humanoid]] spell on Hides-His-Foot before you speak to him and take him away from danger).
Another way to make sure you do not kill any guards is this : Speak to the guard nearest to Hides-His-Foot (HHF) and Taunt him. When he starts attacking, run to the east, while staying inside the plantation walls. Bring him to the very eastern wall and then cast Morrowind:Calm Humanoid|Calm Humanoid]] on him for as long as you can (even just 10 seconds should be sufficient). Then quickly come back to where HHF is hiding. You`ll have to repeat the technique for at least another guard. If the guards come back too soon, taunt them again and bring them further away from HHF. You may have to lead a guard well outside the plantation. When all the guards have been lured away, then you can go talk to HHF for him to follow you.
Escort the slave to the Sterdecan's Farm to the northeast of the plantation and have the farmer take him (he must be taken to the door of the farmhouse for your journal to be updated).