You'll find the slave outside in the plantation, though if the guards see you free him they will attack you. You can try luring the guards away before freeing him. Use a [[Morrowind:Command Humanoid|Command Humanoid]] spell to bring them to the easternmost wall, or taunt them into attacking you, force them to chase you to the eastern wall then cast [[Morrowind:Calm Humanoid|Calm Humanoid]]. Once all the guards are out of the way, you can free Hides-His-Foot without being challenged. Alternatively, you can use a Command spell on Hides-His-Foot to move him away from danger, or simply kill the guards first.
Another bloodless way is to levitate on the mushroom that's right outside the plantation wall and hit him once with an arrow, then jump right back down. He 'll start running but wait where you are, he 'll come back and start punching you. Check no guard saw you and start running towards Sterdecan's farm. Once away from the plantation, use a calm humanoid spell on him and talk to him. He 'll follow you like nothing happened.
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).