Barkley's position was always love Everton, hate Koeman. Koeman was threatening to sell him, imagine him saying that to Lukaku as a means to get him to stay.
He didn't really threaten him. He said if Barkley didn't sign the contract, which koeman actually said he wanted him to, then he'd be forced to sell him and not let him just walk for nothing the next season which would have made all of us even more livid.
I really don't get where this myth of koeman not liking Ross and treating him terribly has come from. Koeman had always said Ross was a good player, and always said he wanted Ross to stay.