The laws on the books for the State of Illinois. 1856.

(206.) Sec. CLX. If any person or persons entitled to the service or labor of any negro, mulatto or colored person, by indenture or other contract or registry made or entered into under the laws of the late territory of Indiana or of Illinois, having a right to hold such person of color in temporary servitude, by virtue of those laws and the constitution of this State, shall hire out, or send any such negro, mulatto or colored person, or any of his or her children, to live or reside in any other State, territory or country, or shall cause, procure or suffer it to be done, or shall sell or otherwise dispose of any such person of color, or the children of such, for the purposes aforesaid, to any citizen or resident of another State, territory or country, before the expiration of his or her term of service, every person so offending, and all purchasers of such colored persons so sold or removed, shall forfeit and lose all right and title or claim to the service of such person of color, and shall, on conviction, for each offense be fined not exceeding five hundred dollars, one-half to be applied to the use of the person injured, and the other half to the use of the county.

Slaves in Illinois, after the anti-slavery laws, could be changed to "indentured servants" for a term of 99 years, if properly registered in said county of the prior Territory or later State of Illinois.

Abraham Lincoln debated Douglas over this issue and publically denied that slavery EVER existed in the State of Illinois-- whereas Douglas stated the actual history and condition of slavery in that State...which did exist after 1847 under a different name... indentured servitude. The French peoples of that State who lived and owned slaves in that territory long before it was ceded to the United States virtually ignored the Northwest Territory laws and continued to own their slaves even into State hood without much fear of that law.

So, you may have something there.