Slavery is being forced to work against your will for the benefit of your master, your owner. The only reason they own the products of your labor is because they own you. If you had exclusive control over what is done to and with your body, the most basic right of self-ownership, you would not owe anybody your labor.
So then a lesser percentage of forced labor is also slavery, though to an obviously lesser degree.
Whether a cent or a million dollars is taken without consent, it is theft. And if someone forces you to work for them 1% of the time, or 100% of a time, that is still slavery.
Frederick Douglass was a slave, by any reckoning. So it is interesting to read his own words, on having his rightfully earned wages taken by force. In his book My Bondage and My Freedom, Douglass laments the state of his servitude, that all his hard work is confiscated from him.
Besides, I was now getting—as I have said—a dollar and fifty cents per day. I contracted for it, worked for it, earned it, collected it; it was paid to me, and it was rightfully my own; and yet, upon every returning Saturday night, this money—my own hard earnings, every cent of it—was demanded of me, and taken from me by Master Hugh. He did not earn it; he had no hand in earning it; why, then, should he have it? I owed him nothing.