Hey @jeremiahscanlon - this is a good question - the new fulfillment API can be a little tricky in terms of understanding functionality since it’s fairly multilayered. At the moment, if the inventory quantity for a fulfillment has to be adjusted, this would require for the initial fulfillment to be cancelled and then re-requested with the correct inventory amount by the merchant or their inventory management app. If you’re building a fulfillment service app, there’s a good run down of how the process works from that perspective here in the dev docs.
Hope this helps!
Al | Shopify Developer Support