Not very positive, but the above is all I have to go on. Unless, someone else knows different, and I'm sure they'll contribute if they do.
Thanks Dennis, I referred to the command-line syntax prior to my scripting in my file manager, as the topic suggested, the issue is about <item>:
When I feed df with file manager's native variable called <seltiems>, df likes it, I guess that's because the "<>" format is what df likes, however, in order to do something more flexible, I have to use a user-defined variable, indicated by the "$" character. Then df doesn't like it anymore.
But that's my guess, I'm still very confused, because that's just a variable, it should be resolved to its value...