diff --git a/lib/spree/localized_number.rb b/lib/spree/localized_number.rb
index a1ba18accd..f972399b0a 100644
--- a/lib/spree/localized_number.rb
+++ b/lib/spree/localized_number.rb
@@ -45,7 +45,7 @@ module Spree
     def self.valid_localizable_number?(number)
       return true unless number.is_a?(String) || number.respond_to?(:to_d)
       # Invalid if only two digits between dividers, or if any non-number characters
-      return false if number.to_s =~ /[.,]\d{2}[.,]/ || number.to_s =~ /[^0-9,.]+/
+      return false if number.to_s =~ /[.,]\d{2}[.,]/ || number.to_s =~ /[^-0-9,.]+/
 
       true
     end
diff --git a/spec/lib/spree/localized_number_spec.rb b/spec/lib/spree/localized_number_spec.rb
index bafe0c822d..b19df7e7ef 100644
--- a/spec/lib/spree/localized_number_spec.rb
+++ b/spec/lib/spree/localized_number_spec.rb
@@ -58,6 +58,12 @@ describe Spree::LocalizedNumber do
       it "returns true" do
         expect(described_class.valid_localizable_number?(1599.99)).to eql true
       end
+
+      context "with a negative number" do
+        it "returns true" do
+          expect(described_class.valid_localizable_number?(-1599.99)).to eql true
+        end
+      end
     end
 
     context "with letters" do
diff --git a/spec/system/admin/adjustments_spec.rb b/spec/system/admin/adjustments_spec.rb
index a88b0b4f15..f1a181561f 100644
--- a/spec/system/admin/adjustments_spec.rb
+++ b/spec/system/admin/adjustments_spec.rb
@@ -23,6 +23,10 @@ describe '
                       zone: create(:zone_with_member), tax_category: tax_category)
   }
 
+  let!(:tax_category_included) { create(:tax_category, name: 'TVA 20%', is_default: true) }
+  let!(:default_tax_zone) { create(:zone, default_tax: true) }
+  let!(:tax_rate2) { create(:tax_rate, name: "TVA 20%", amount: 0.2, zone: default_tax_zone, included_in_price: true, tax_category: tax_category_included, calculator: Calculator::DefaultTax.new ) }
+
   before do
     order.finalize!
     create(:check_payment, order: order, amount: order.total)
@@ -30,22 +34,78 @@ describe '
     visit spree.admin_orders_path
   end
 
-  it "adding taxed adjustments to an order" do
-    # When I go to the adjustments page for the order
-    page.find('td.actions a.icon-edit').click
-    click_link 'Adjustments'
+  shared_examples "when the enable_localized_number preference" do |adjustment_label, adjustment_amount, tax_category, tax, tax_total|
+    it "creates the adjustment and calculates taxes" do
+      # When I go to the adjustments page for the order
+      page.find('td.actions a.icon-edit').click
+      click_link 'Adjustments'
 
-    # And I create a new adjustment with tax
-    click_link 'New Adjustment'
-    fill_in 'adjustment_amount', with: 110
-    fill_in 'adjustment_label', with: 'Late fee'
-    select 'GST', from: 'adjustment_tax_category_id'
-    click_button 'Continue'
+      # And I create a new adjustment with tax
+      click_link 'New Adjustment'
+      fill_in 'adjustment_amount', with: adjustment_amount
+      fill_in 'adjustment_label', with: adjustment_label
+      select tax_category.to_s, from: 'adjustment_tax_category_id'
+      click_button 'Continue'
 
-    # Then I should see the adjustment, with the correct tax
-    expect(page).to have_selector 'td.label', text: 'Late fee'
-    expect(page).to have_selector 'td.amount', text: '110.00'
-    expect(page).to have_selector 'td.tax', text: '10.00'
+      # Then I should see the adjustment, with tax included in the amount
+      expect(page).to have_selector 'td.label', text: adjustment_label.to_s
+      expect(page).to have_selector 'td.amount', text: adjustment_amount.to_s
+      expect(page).to have_selector 'td.tax-category', text: tax_category.to_s
+      expect(page).to have_selector 'td.tax', text: tax.to_s
+      expect(page).to have_selector 'td.total', text: tax_total.to_s
+    end
+  end
+
+  context "is active" do
+    before do
+      allow(Spree::Config).to receive(:enable_localized_number?).and_return(true)
+    end
+
+    context "included tax" do
+      context "adding negative, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Discount", "-2", "TVA 20%", "$0.33", "$-1.67"
+      end
+
+      context "adding positive, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Late fee", "100", "TVA 20%", "$-16.67", "$83.33"
+      end
+    end
+
+    context "added tax" do
+      context "adding negative, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Discount", "-2", "GST", "$10.00", "$8.00"
+      end
+
+      context "adding positive, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Late fee", "110", "GST", "$10.00", "$120"
+      end
+    end
+  end
+
+  context "is not active" do
+    before do
+      allow(Spree::Config).to receive(:enable_localized_number?).and_return(false)
+    end
+
+    context "included tax" do
+      context "adding negative, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Discount", "-2", "TVA 20%", "$0.33", "$-1.67"
+      end
+
+      context "adding positive, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Late fee", "100", "TVA 20%", "$-16.67", "$83.33"
+      end
+    end
+
+    context "added tax" do
+      context "adding negative, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Discount", "-2", "GST", "$10.00", "$8.00"
+      end
+
+      context "adding positive, taxed adjustments to an order" do
+        it_behaves_like "when the enable_localized_number preference", "Late fee", "110", "GST", "$10.00", "$120"
+      end
+    end
   end
 
   it "modifying taxed adjustments on an order" do
@@ -105,8 +165,8 @@ describe '
     it "displays adjustments" do
       click_link 'Adjustments'
 
-      expect(page).to_not have_selector('tr a.icon-edit')
-      expect(page).to_not have_selector('a.icon-plus'), text: 'New Adjustment'
+      expect(page).to_not have_selector 'tr a.icon-edit'
+      expect(page).to_not have_selector 'a.icon-plus', text: 'New Adjustment'
     end
   end
 end