As mentioned in our dashboard, inventory is handled the following way during an exchange process:
Variant-Exchange during the customer's return process (PRO Plan and up)
when the exchange order is created from our dashboard
inventory is decremented for the new items
original items can be restocked in the same dialog in our dashboard or later individually in our dashboard (e.g., when one of the items being returned cannot be resold)
One-Click Draft-Order (PREMIUM Plan and up)
when the draft exchange order is created from our dashboard
Shopify does not allow reserving inventory for draft-orders
the inventory is taken when the exchange draft order is moved to an actual order
original items can be restocked in the same dialog in our dashboard or later individually in our dashboard (e.g., when one of the items being returned cannot be resold)