In case you haven't checked the "Buy Slave" spell is not a once per turn spell; you can cast it as many times as you want with a single unit, which means buying slaves with slaves isn't really broken, when you can buy hundreds with a single warrior anyway. But that lowers hurrying prices a lot, that could be broken. (I mean buying slaves and sacrificing them to hurry stuff.)
Actually in the game i'm playing now (.31d), each unit can only buy 1 slave per round...but new slaves can buy another and so on.
In version .30 I remember being able to buy multiple slaves with one unit, so I think it was changed.