diff --git a/core/app/models/spree/fulfilment_changer.rb b/core/app/models/spree/fulfilment_changer.rb
index 61a4234716f..0884fa14096 100644
--- a/core/app/models/spree/fulfilment_changer.rb
+++ b/core/app/models/spree/fulfilment_changer.rb
@@ -86,9 +86,9 @@ def run!
     # we can take from the desired location, we could end up with some items being backordered.
     def run_tracking_inventory
       # Retrieve how many on hand items we can take from desired stock location
-      available_quantity = [desired_shipment.stock_location.count_on_hand(variant), default_on_hand_quantity].max
-
+      available_quantity = get_available_quantity
       new_on_hand_quantity = [available_quantity, quantity].min
+      backordered_quantity = get_backordered_quantity(available_quantity, new_on_hand_quantity)
       unstock_quantity = desired_shipment.stock_location.backorderable?(variant) ? quantity : new_on_hand_quantity
 
       ActiveRecord::Base.transaction do
@@ -105,20 +105,22 @@ def run_tracking_inventory
         # These two statements are the heart of this class. We change the number
         # of inventory units requested from one shipment to the other.
         # We order by state, because `'backordered' < 'on_hand'`.
+        # We start to move the new actual backordered quantity, so the remainder
+        # can be set to on_hand state.
         current_shipment.
           inventory_units.
           where(variant: variant).
           order(state: :asc).
-          limit(new_on_hand_quantity).
-          update_all(shipment_id: desired_shipment.id, state: :on_hand)
+          limit(backordered_quantity).
+          update_all(shipment_id: desired_shipment.id, state: :backordered)
+      end
 
         current_shipment.
           inventory_units.
           where(variant: variant).
           order(state: :asc).
-          limit(quantity - new_on_hand_quantity).
-          update_all(shipment_id: desired_shipment.id, state: :backordered)
-      end
+          limit(quantity - backordered_quantity).
+          update_all(shipment_id: desired_shipment.id, state: :on_hand)
     end
 
     # When we don't track inventory, we can just move the inventory units from one shipment
@@ -141,11 +143,27 @@ def handle_stock_counts?
       current_shipment.order.completed? && current_stock_location != desired_stock_location
     end
 
-    def default_on_hand_quantity
+     def get_available_quantity
+      positive_stock = -> (shipment) do
+        on_hand = shipment.stock_location.count_on_hand(variant)
+        on_hand.positive? ? on_hand : 0
+      end
+
+      if current_stock_location != desired_stock_location
+        positive_stock.call(desired_shipment)
+      else
+        sl_availability = positive_stock.call(current_shipment)
+        shipment_availability = current_shipment.inventory_units.where(variant: variant).on_hand.count
+        sl_availability + shipment_availability
+      end
+    end
+
+    def get_backordered_quantity(available_quantity, new_on_hand_quantity)
       if current_stock_location != desired_stock_location
-        0
+        quantity - new_on_hand_quantity
       else
-        current_shipment.inventory_units.where(variant: variant).on_hand.count
+        shipment_quantity = current_shipment.inventory_units.where(variant: variant).size
+        shipment_quantity - available_quantity
       end
     end