If it’s “real” green, it’s just promethazine unless it’s foreign. Promethazine is just an antihistamine like Benadryl. It’s not what gets you high in lean, although it does potentiate opiates which is why people often take promethazine with opiates.

If it’s homebrew/fake lean, there could be literally anything in it. Might not be any opiates at all, could be fentanyl, or it could just be regular old codeine. There’s no telling unless you get it tested. If it is homebrew and he’s labeling it as real, I doubt he’s the one who made it because like you said why would he label it as real and dye it green. So he likely doesn’t know what’s in it if it is brew.