Was the slave free at that point since the state didn't recognize slavery? I know Dred Scott ruled that slaves in free territories were still considered slaves. Also the fugitive slave act seems to imply that slaves were not freed by being in a free state.
Edit: the heart of my question is what prevented a white slave holder from moving to a free state with his slaves?
I'm not 100% sure I'm understanding your question, but I believe you're asking if a slave was still a slave if the person who owned them moved to a free state and brought them along. In that case, the slave would be free since slavery is illegal in the state that they moved to. I can't imagine a situation where a slave would have to be freed by the state due to a slave owner moving to a free state though. The slave owner would have either freed his slaves prior to moving, or even more likely he would have sold them to another slave owner.
If a slave owner was visiting a free state and decided to bring one of his slaves with him the slave would have still been the property of the slave owner regardless of the states laws. Slavery wasn't illegal under federal law, it was just outlawed by certain states. I really doubt a slave owner would have been bothered by most people in free states, even if he had one of his slaves with him. Hardcore abolitionists didn't make up a large segment of the population anywhere in the US really until the Civil War was already underway.