Hi Mark,

I think you can achieve this by giving a onchage function for  that field, inside the onchange function you can check whether the journal is the exact one or not. If yes you can continue.

Thank you

@api.onchange('journal_id')
def check_journal(self):
    if self.journal_id and self.journal_id.id == 1:
        # here you can write the function body
        # make necessary change in if condition, its just a sample