Hey @edsgroup
The thread you linked to is the older API which is being depreciated next year. If you’re referring to the newer REST API, this issue was reported a few days ago and I’ve just posted an update in that thread (Purchase Reference and Credit Note - #7 by QFMathew).
Once the issue in that linked post is resolved, I will send over an update for relating to the quantity and unit cost variables.
Hope that helps, but please let me know if you have aby further questions.