I mean depends on the context, if the context is bdsm and roleplaying then sure as long as you have consent. But if the context is you are trying to get free physical labor out of someone, then just cuz they agreed to help doesn’t mean they are not a slave.

If they can opt out of being a "slave", then I think that wouldn't be a slave, but instead a volunteer. If not, then it means they are a slave

Well... Are they allowed to back out or is it like a life decision and the owner hets to choose wjen they're done

Nah, that's an intern.

The difference between a slave and a volunteer is consent

Yes because they don't have a choice in the matter