When we are slaves to sin, we are self-willed, self-determined, self-sufficient etc. By continuing to live for ourselves, we are choosing not to live for Him that died for all and rose again. That is an act of volation; an expression of free will.
That's how a sinner can be a slave, bound by the chains of sin, while simultaneously exercising free will, preferring to be god over their own life, rather than choosing the life God would have us all live.
Free will is the power to make a decision independently. We either live in wilful ignorance, falling in favour of our flesh, or we live by faith in the Son of God, who loves us and gave His life for us.
One choice; no alternatives. We are given the freedom to choose, but with there being no alternative, other than the flesh, or the Spirit, we are figuratively shackled/restrained, like a slave, compelled, with no smorgasbord of options to pick from.
You seem to think "free will" and choice are one in the same and I reject. I disagree on this completely because our choice is always going to be limited by out nature, be that in sin, or in the Spirit. No choice you make before you're born again can please God. Paul tells us no man CAN please God. Then once He gives us grace and spiritually resurrects us and reconciles us to His Spirit it's Him that empowers us to make the choices that honor Him, either way our choices are still a slave to our nature. So the term "free will" is just not accurate ever. we NEVER have a "FREE" will according to God. You can disagree, but you're disagreeing with God. I personally don't think that's wise. Choice does NOT= free will.