Use the order outstanding balance to create payment

When a customer credit is applied to an order, the balance due is the
order outstanding balance and not the order total.

app/services/checkout/params.rb

@@ -61,7 +61,7 @@ module Checkout
     def set_payment_amount
       return unless @order_params[:payments_attributes]
 
-      @order_params[:payments_attributes].first[:amount] = order.total
+      @order_params[:payments_attributes].first[:amount] = order.outstanding_balance.amount
     end
 
     def set_existing_card

engines/order_management/app/services/order_management/order/updater.rb

@@ -259,7 +259,7 @@ module OrderManagement
 
       def update_payment
         # Update payment with correct amount
-        @payment.update_attribute :amount, order.total
+        @payment.update_attribute :amount, order.outstanding_balance.amount
       end
 
       def update_payment_adjustment(amount)

engines/order_management/spec/services/order_management/order/updater_spec.rb

@@ -155,11 +155,10 @@ RSpec.describe OrderManagement::Order::Updater do
       end
 
       context "with pending payments" do
+        let(:order) { create(:order_with_totals, state: "payment") }
         let!(:payment) { create(:payment, order:, amount: order.total) }
 
         context "with order in payment state" do
-          let(:order) { create(:order_with_totals, state: "payment") }
-
           it "updates pending payments" do
             # update order so the order total will change
             update_order_quantity(order)
@@ -167,6 +166,16 @@ RSpec.describe OrderManagement::Order::Updater do
 
             expect { updater.update }.to change { payment.reload.amount }.from(10).to(20)
           end
+
+          context "with mutiple payments" do
+            it "updates pending payments" do
+              create(:payment, order:, amount: 5.00, state: "completed")
+              last_payment = create(:payment, order:, amount: 10.00)
+              order.payments.reload
+
+              expect { updater.update }.to change { last_payment.reload.amount }.from(10).to(5)
+            end
+          end
         end
 
         context "with order in confirmation state" do

spec/controllers/checkout_controller_spec.rb

@@ -503,6 +503,37 @@ RSpec.describe CheckoutController do
           end
         end
 
+        describe "with customer credit" do
+          let(:credit_payment_method) {
+            order.distributor.payment_methods.customer_credit
+          }
+
+          before do
+            order.customer = create(:customer, enterprise: distributor)
+            order.save!
+          end
+
+          it "adds payment to cover the remaining balance" do
+            # Add credit
+            create(
+              :customer_account_transaction,
+              amount: 100.00,
+              customer: order.customer,
+              payment_method: credit_payment_method
+            )
+            # Create credit payment
+            payment = order.payments.create!(payment_method: credit_payment_method, amount: 3.00)
+            payment.internal_purchase!
+
+            put(:update, params:)
+
+            expect(response).to redirect_to checkout_step_path(:summary)
+            last_payment = order.payments.last
+            expect(last_payment.amount).to eq(7.00)
+            expect(last_payment.payment_method.id).to eq(payment_method.id)
+          end
+        end
+
         context "with insufficient stock" do
           it "redirects to details page" do
             allow(order).to receive_message_chain(:insufficient_stock_lines,

spec/system/consumer/checkout/summary_spec.rb

@@ -453,10 +453,13 @@ RSpec.describe "As a consumer, I want to checkout my order" do
               # actual order total is 10.00
               expect(page).to have_selector("#order_total", text: with_currency(8.00))
             end
+
+            last_payment = order.payments.last
+            expect(last_payment.amount).to eq(8.00) # it is the balance due
           end
 
           context "when completing order" do
-            it "displays part of the order whas paid with credit" do
+            it "displays part of the order that was paid with credit" do
               # Move to ready for confirmation
               order.next!