curl --request POST \
  --url https://api.headlesscommerce.io/v1/admin/inventory/{item_id}/adjust \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "quantity": 123,
  "reason": "<string>"
}
'

{
  "id": "<string>",
  "variant_id": "<string>",
  "track_inventory": true,
  "allow_backorder": true,
  "safety_stock": 123,
  "on_hand": 123,
  "reserved": 123,
  "available": 123,
  "metadata": {}
}