Slavery involves no pay and being unable to leave under threat of violence. Undocumented workers are paid and are free to leave. Indeed, many people are actively asking them to leave.

I take your point, but it's not slavery. A basic income isn't meant to preclude anyone from working. If it existed and undocumented workers were removed, those jobs would have to pay a wage sufficient to entice someone who doesn't need them for survival, or we'd have to go without those services.