While you could use "impairment" here, for Abwertung, as inventories are almost always current assets, the "normal" term used to for such short-term assets is "to write down" (verb) or "write-down" (noun).

I'm am a bit unsure what the author means by Abwertungsroutine, but I would hazard to guess it could simply be the "scheduled/normal impairment testing" rather than some "scheme" for calculating the amount of the necessary write-down, i.e., the accounting policy for remeasuring inventory and not the actual calculation.