I think you have your past and future mixed up. Precedent (also known as “stare decisis”) is reliance on an issue that has already been decided. A “slippery slope” argument makes an argument for a circumstance that hasn’t happened, and is unlikely to happen due to the fact that it’s not logically connected with the issue.
No, I have it just right, Whitey.
Consider, for example, a hypothetical Bill-of-Rights-eroding law designed to make it easier to arrest terrorists, or drug dealers, or pedophiles. The “slippery slope” argument says that, ultimately, such heinous laws will ultimately be used against ALL OF US, so be dang sure you can live with the law applied to you, personally, before supporting it. “Legal precedent” is the mechanism by which the “slippery slope” is attained. “Equal protection under the law” says that we can’t have laws that only are applied to “really bad dudes”; they apply to all. Which is why the ACLU spends so much time defending scumbags–they know such laws will ultimately affect the rest of us.