diff --git a/core/app/models/spree/fulfilment_changer.rb b/core/app/models/spree/fulfilment_changer.rb
index 747cb86f5a4..31c8a99b15e 100644
--- a/core/app/models/spree/fulfilment_changer.rb
+++ b/core/app/models/spree/fulfilment_changer.rb
@@ -43,17 +43,21 @@ def run!
       return false if invalid?
       desired_shipment.save! if desired_shipment.new_record?
 
-      new_on_hand_quantity = [desired_shipment.stock_location.count_on_hand(variant), quantity].min
+      # Retrieve how many on hand items we can take from desired stock location
+      available_quantity = [desired_shipment.stock_location.count_on_hand(variant), 0].max
+
+      new_on_hand_quantity = [available_quantity, quantity].min
+      unstock_quantity = desired_shipment.stock_location.backorderable?(variant) ? quantity : new_on_hand_quantity
 
       ActiveRecord::Base.transaction do
         if handle_stock_counts?
           # We only run this query if we need it.
-          current_on_hand_quantity = [current_shipment.inventory_units.on_hand.size, quantity].min
+          current_on_hand_quantity = [current_shipment.inventory_units.pre_shipment.size, quantity].min
 
           # Restock things we will not fulfil from the current shipment anymore
           current_stock_location.restock(variant, current_on_hand_quantity, current_shipment)
           # Unstock what we will fulfil with the new shipment
-          desired_stock_location.unstock(variant, new_on_hand_quantity, desired_shipment)
+          desired_stock_location.unstock(variant, unstock_quantity, desired_shipment)
         end
 
         # These two statements are the heart of this class. We change the number
diff --git a/core/spec/models/spree/fulfilment_changer_spec.rb b/core/spec/models/spree/fulfilment_changer_spec.rb
index eca50082e19..eaec960b7ec 100644
--- a/core/spec/models/spree/fulfilment_changer_spec.rb
+++ b/core/spec/models/spree/fulfilment_changer_spec.rb
@@ -110,12 +110,12 @@
           stock_item.update(backorderable: true)
         end
 
-        it "restocks five at the original stock location" do
+        it "restocks seven at the original stock location" do
           expect { subject }.to change { current_shipment.stock_location.count_on_hand(variant) }.by(7)
         end
 
-        it "unstocks five at the desired stock location" do
-          expect { subject }.to change { desired_shipment.stock_location.count_on_hand(variant) }.by(-5)
+        it "unstocks seven at the desired stock location" do
+          expect { subject }.to change { desired_shipment.stock_location.count_on_hand(variant) }.by(-7)
         end
 
         it "creates a shipment with the correct number of on hand and backordered units" do
@@ -124,6 +124,24 @@
           expect(desired_shipment.inventory_units.backordered.count).to eq(2)
         end
 
+        context "when the desired stock location already has a backordered units" do
+          let(:desired_count_on_hand) { -1 }
+
+          it "restocks seven at the original stock location" do
+            expect { subject }.to change { current_shipment.stock_location.count_on_hand(variant) }.by(7)
+          end
+
+          it "unstocks seven at the desired stock location" do
+            expect { subject }.to change { desired_shipment.stock_location.count_on_hand(variant) }.by(-7)
+          end
+
+          it "creates a shipment with the correct number of on hand and backordered units" do
+            subject
+            expect(desired_shipment.inventory_units.on_hand.count).to eq(0)
+            expect(desired_shipment.inventory_units.backordered.count).to eq(7)
+          end
+        end
+
         context "when the original shipment has on hand and backordered units" do
           before do
             current_shipment.inventory_units.limit(6).update_all(state: :backordered)
@@ -137,16 +155,21 @@
         end
 
         context "when the original shipment had some backordered units" do
+          let(:current_stock_item) { current_shipment.stock_location.stock_items.find_by(variant: variant) }
+          let(:desired_stock_item) { desired_shipment.stock_location.stock_items.find_by(variant: variant) }
+          let(:backordered_units)  { 6 }
+
           before do
-            current_shipment.inventory_units.limit(6).update_all(state: :backordered)
+            current_shipment.inventory_units.limit(backordered_units).update_all(state: :backordered)
+            current_stock_item.set_count_on_hand(-backordered_units)
           end
 
           it "restocks four at the original stock location" do
-            expect { subject }.to change { current_shipment.stock_location.count_on_hand(variant) }.by(4)
+            expect { subject }.to change { current_stock_item.reload.count_on_hand }.from(-backordered_units).to(1)
           end
 
           it "unstocks five at the desired stock location" do
-            expect { subject }.to change { desired_shipment.stock_location.count_on_hand(variant) }.by(-5)
+            expect { subject }.to change { desired_stock_item.reload.count_on_hand }.from(5).to(-2)
           end
 
           it "creates a shipment with the correct number of on hand and backordered units" do