Hello Kai,
yes, you are correct and I am aware about this so I extra write in my post "when we want to follow Volz".
But frankly said sounds halasan very similar to kalasan, it may be just the different dialect/language between this both Batak groups or the different understanding/translation from Volz and van der Tuuk. It just would be good to know the exact translation of kalasan/halasan.
But agree with you for sure that halasan would be a better denotation that expert collectors understand direct about what we speak.
Regards,
Detlef