The obedience requirement is an alright idea, it would limit the amount of MUP gained per day from capturing fresh slaves. Though I'm not fond of the MUP system, as it adds an arbitrary mechanic to the game that has no grounding or presence in the game world. I feel that there should be a better system for encouraging players to capture slaves.
I'm a little confused about your problem with "sell production for gold". This option only has effect when enabled with the slave in the farm at the end of the day with a farm manager assigned. Slaves in the farm produce either food or good depending on the option. Releasing the slave from the farm would guarantee that it did nothing.