[Answered ]-How can I get the incoming value of a property that is calculated from related objects in a model's clean method

You can try like this in the LineItem model form (explanation in code comments):

class SomeFormItem(forms.ModelForm):
   ...
   def save(self, commit=False):
       instance = super().save(commit=False)
       total = instance.invoice.total  #  get total of line items (assuming used using reverse query)

       if self.instance.pk and ('quantity' in self.changed_data or 'unit_price' in self.changed_data):  # if we are editing already existing lineitem and total has been changed
           total += (self.cleaned_data['unit_price'] - self.data['unit_price']) * (self.cleaned_data['quantity'] - self.data['quantity'])  # self.cleaned_data contains new information and self.data contains old information and calculating the difference only
       else:
           total += self.cleaned_data['quantity'] * self.cleaned_data['unit_price']

       if total  > SOME_NUMBER:
            raise ValidationError()
       return super().save(commit=commit)

More information can be found in the documentation.