If I remember correctly criminal slaves go free if they ern enough money through work while Shiro and Wendy is his forever? It's hard to remember specifics considering they are fapping around, but wasn't the adventurer girls supposed to buy themselves out of slavery by doing quests and collecting materials for MC's alchemy but they did it like once so far?
At first Aina and Sorte were just debt slaves due to the dumbasses he met early in the manga and Aina took responsibility for the guild's treatment of him.
But Sorte being an immature brat lied about MC to Renge and she not only caused a large amount of damage but nearly killed him.
Due to that her and for Sorte's role in it were made criminal slaves. Aina is still just a debt slave (BTW their debt is huge yet still not close to the price MC paid for Wendy) and will be free once it's paid (which includes the damage done by Renge) but she's pretty much willing to stay his slave.
Sorte and Renge are lifelong slaves of his iirc due to their crimes. Wendy and Shiro are pretty much free slaves, as they can act independently from him.