As mentioned in our dashboard, inventory is handled the following way during an exchange process:
your live inventory is checked for availability
NO inventory is reserved at this point
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)
Related: Are Exchange Orders created automatically?
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)
Related: Are Exchange Orders created automatically?