They're slaves because they are prohibited from bettering their circumstances. Now, if you abolish the slavery and then say "You're free now! Go better yourself

They're slaves because they are prohibited from bettering their circumstances. Now, if you abolish the slavery and then say "You're free now! Go better yourself!" without giving them any resources with which to do so - if they lack the tools to feed themselves - they'll starve, yes. To better themselves, they need to be able to make something where they're at, or be able to go somewhere else. If they're sitting on ruined land, unable to leave, that's a problem that can't be solved with mere hard work. But that's a problem that would not have happened without the slavery in the first place.