Imagine a society (medieval, but that isn't very important here) that has approved slavery. You can trade, buy, sell and hire slaves at will in the market.
Slaves are usually "harvested" from indigenous villages that are attacked and conquered by the kingdom. Also, but not so common, people can sell their sons or even themselves to be slaves, usually if they are extremely poor (at least their new owners will usually take care of them, otherwise they will lose money). Another way to get slaves is during a war, war prisoners sometimes aren't returned to their homes after a war and they become slaves of the victorious empire, even this may happen with civilians in war zones.
The idea: Sell prisoners.
The thing that I want to add is the possibility of buying prisoners as slaves. This will have 2 main advantages:
- The government will spend less money on prisons.
- The slave market will have cheaper prices due a new and constant flow of slaves to the market (wars and indigenous villages aren't so common).
And finally, it may be a bit interesting.
It's quite understandable and obvious that slaves can be released. An owner can at will give freedom to its slaves, he just needs to sign a little contract that states the freedom of the person. Sometimes slaves perform deals with they masters if they work hard and without causing problems during X years they get freedom.
If you can buy prisoners as slaves and you can get them free at will, you can, as a rich person, member of a rich family, assassins/thief guild, etc, perform a criminal activity, be jailed, be bought by a friend, and finally be released by him.
Basically, the law won't be any problem for rich or illegal organizations.
How can I prevent this problem but allow the sale of prisoners?
Even not allowing slaves to be set free by their masters it still being the same. You can be bought by a friend, family or guild and keep your life as normal. I don't want that.
Not much necessary to know
In this world, there is magic, some magic (usually healing magic) need life forces of victims to be performed. I want to let mages buy cheap prisoners and use them as life-force batteries to power they healing services in rich hospitals. I was thinking in prisoners because they are actually criminals, not just "slaved persons", so mages wouldn't annoy about kill them to fuel their magic, also, they may be cheaper.
A solution could be to not buy the prisoner, just its life-force. A mage visits a prison, ask for some life-force, pay the price, guards transfer the prisoner to a special room, and there part of its life is taken away from him to a jar by the mage. Simple, easy and effective. But I don't like much the idea. I would like the idea of buy prisoners as assets, not just their life essence.