Lily has a debt and will become a slave if she can't pay it.
By law, no one can touch another's slave, regardless of station.
You can willingly become a slave.
Don't know if this is the direction Cri-ten is going for. But it seems obvious. And it can be a smart way to give the player a reason to collect currency.
If a slave has debt before willingly becoming a slave, the debt is transferred to the master. Should the master become unable to pay the debt. The master and all their slaves become slaves of the person who's owed the debt.
Haven't gotten too far in yet(had to restart thanks to the bug explained by another below). But if this is the way Cri-ten is going. I'm impressed.