All workers under capitalism are slaves in a loose sense of the word. Your labor creates more profit than what they pay you in wages (otherwise the owners wouldn't employ anyone). Typically, your wages are only a small fraction of the money your labor makes the owner.

While the capitalist gets to pick a profitable time in which to invest their money (e.g., buy labor, machines, stocks, etc.) the worker is born into institutions that force them, on threat of destitution, to sell themselves by the hour. We are really not much different from feudal serfs.