I’m afraid I only know a little on this one, but I can offer an article!

https://today.usc.edu/how-mexico-offered-freedom-to-the-enslaved-people-of-the-antebellum-south

Mexico had banned slavery in 1837, long before its northern neighbor. The nation’s laws granted enslaved people access to freedom as soon as they set foot on Mexican soil. Mexican officials also refused to return runaways under the U.S.’s Fugitive Slave Act.