Paralyze Harkon (Ice Form works well for this), then while he's in the process of standing up (not while he's still on the floor), take it from him. It'll say 0% still, but it'll work anyway. Alternatively, if you aren't a fan of using glitches, just wear stuff with fortify pickpocketing and use pickpocketing potions and hope for the best, or simply wait until you can kill him for it.
Paralyze Harkon (Ice Form works well for this), then while he's in the process of standing up (not while he's still on the floor), take it from him. It'll say 0% still, but it'll work anyway. Alternatively, if you aren't a fan of using glitches, just wear stuff with fortify pickpocketing and use pickpocketing potions and hope for the best, or simply wait until you can kill him for it.