The Bible never describes the Israelites as "slaves."
The Israelites perform forced labor for Pharaoh, and they have Egyptian taskmasters who "ruthlessly imposed" on them. But they don't seem to be slaves in the way Joseph was a slave to Potiphar. The Israelites aren't owned by Egyptians. There appear to be limits on their maltreatment: They are compelled to supply labor, but there's no mention of them being deprived of property or banned from other work. The word "slave," which is thrown about so casually everywhere else in the Bible doesn't appear in reference to the Israelites .