its not full freedom, but its dam close. Its not good, but its better than the alternative they would face.
Lesser of two evils, that kind of things.
Obviously, it's better if the slave is treated humanely, but that is still not freedom by a long shot.
Your owner will treat you humanely? That sounds good, but what if you don't like them back? What if they give you orders you object to? You don't have much of a choice then. If you don't obey, said owner might change their mind and either return you or treat you... less humanely. This is not a relationship as equal, no matter how positive a light you shine on this.
And that's not even factoring sexual activities in the equation. At which stage, it crosses another level of evil.
More importantly, as long as you're a slave, your owner might change for many reasons. Death of the owner, confiscation as a compensation for debts, etc. As long as you're property, you're very far from freedom and just a step away from abuse. At best, you're safe from harm for a while.
Oh, going a little meta, your owner is a manga protagonist? Better hope it's the right genre... and it sticks to it.
This is all about as close to freedom as walking on the very edge of a cliff is close to safety.