This seems a bit unfair to penalize both him and I.  That's 30,000 gold you're asking for, just so I can operate on equal footing as a playing character.  Granted I chose to play a slave, BUT if that was to be so- there was NO reason to deny me the Oath of Poverty benefits because while my concept may be a slave, as a player I should be on equal footing with other players.  And Saric shouldn't be penalized for trying to be a good player and agreeing to purchase my character.  I think at this point you need to establish as a DM how you plan to equalize the situation.  There's a degree of OOC balance that should addressed even though I understand you wish to maintain a sense of 'realism' for a master-slave relationship.  At the very least, maybe offer to let him buy on 'credit' since he is going to have income as the game progresses in time?