I think, and somebody correct me if I am wrong, once a player enters the portal, whatever scholarship they had ends and can be immediately offered to someone else. Of course, a player that enters the portal can come back; whether a scholarship is available is another matter.
Scholarships are actually year to year. Although it's not a good practice IMO, a coach can pull it at the end of any year. In the case of a kid entering the portal the scholarship is at least guaranteed through the end of the current semester but can be offered to other players (portal or freshman) for next season. On the other hand, if they think they have a chance to retain the kid, a coach could theoretically re-recruit him while holding off on awarding those scholarship dollars in case they are successful. I'd imagine that would be a fairly rare instance as I'm not aware of it happening outside of football, where there are a couple of thousand portal entrants per year.