"Many Northern states wanted to circumvent the Fugitive Slave Act. Some jurisdictions passed "personal liberty laws", mandating a jury trial before alleged fugitive slaves could be moved; others forbade the use of local jails or the assistance of state officials in the arrest or return of alleged fugitive slaves. In some cases, juries refused to convict individuals who had been indicted under the Federal law.[2]
The Missouri Supreme Court routinely held with the laws of neighboring free states, that slaves who had been voluntarily transported by their owners into free states, with the intent of the masters' residing there permanently or indefinitely, gained their freedom as a result.[3] The Fugitive Slave Law dealt with slaves who escaped to free states without their master's consent. The U.S. Supreme Court ruled, in Prigg v. Pennsylvania (1842), that states did not have to offer aid in the hunting or recapture of slaves, greatly weakening the law of 1793."
https://en.m.wikipedia.org/wiki/Fugitive_Slave_Act_of_1850