Fix: adjust tax precise amount when issuing credit notes (#2643)

## Context

We have this issue that when we issue Invoice, we apply tax on sum of
items,
but when we issue a credit note, we apply tax on each item separately.
as result, some values can be rounded in deffirent direction which
result in not matching sum.

Example that our client had (all values are amount_cents):
```
Invoice 001
charge01 = 68_33
charge 02 = 68_33
charge03 = 57_50
charge04 = 85_00
```

if we apply 20% taxes on the sum of charges, we get:
```
(68_33 + 68_33 + 57_50 + 85_00) * 1.2 = 334_99,2
```
but because it is in cents, when issuing the invoice, the value is
rounded to `334_99`

Next, when we create credit notes for this invoice, and we create a
separate CN per each item:
```
CN01 = 68_33 * 1.2 = 8199.6 => 82_00
CN02 = 57_50 * 1.2 = 69_00
CN03 = 85_00 * 1.2 = 102_00
CN04 = 68_33 * 1.2 = 8199.6 = 82_00
```
Sum of these credit notes is `335_00` which is more than the invoice
total, and we fail vthe validation, despite the fees amount is correct

## Description

To help this situation, we can operate adjustments that were made when
credit notes are issued.
So when we're creating CN1-3, we simply create credit notes.
when we create CN4, we are subtracting all rounding adjustments that
were made for all credit notes for this invoice. in our case, we will
need to subtract `0.4` cents from precise taxes amount, and then for the
last credit note
`CN04 = 68_33 * 1.2 = 8199.6 -0.4 = 8199.2 => 8199`
and the values are matching

also removed validation on credit_note_item total amount, as it tries to
predict taxes, but they are adjusted further in the credit note creation
service

app/models/credit_note.rb

@@ -129,6 +129,18 @@ def sub_total_excluding_taxes_amount_cents
   end
   alias_method :sub_total_excluding_taxes_amount_currency, :currency
 
+  def precise_total
+    items.sum(&:precise_amount_cents) - precise_coupons_adjustment_amount_cents + precise_taxes_amount_cents
+  end
+
+  def taxes_rounding_adjustment
+    taxes_amount_cents - precise_taxes_amount_cents
+  end
+
+  def rounding_adjustment
+    total_amount_cents - precise_total
+  end
+
   private
 
   def ensure_number

app/models/invoice.rb

@@ -273,7 +273,7 @@ def creditable_amount_cents
       fee_rate = fee.creditable_amount_cents.fdiv(fees_total_creditable)
       prorated_credit_amount = credit_adjustement * fee_rate
       (fee.creditable_amount_cents - prorated_credit_amount) * (fee.taxes_rate || 0)
-    end.fdiv(100).round
+    end.fdiv(100).round # BECAUSE OF THIS ROUND the returned value is not precise
 
     fees_total_creditable - credit_adjustement + vat
   end

app/services/credit_notes/create_service.rb

@@ -205,10 +205,24 @@ def compute_amounts_and_taxes
       credit_note.precise_coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents
       credit_note.coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents.round
       credit_note.precise_taxes_amount_cents = taxes_result.taxes_amount_cents
-      credit_note.taxes_amount_cents = taxes_result.taxes_amount_cents.round
+      adjust_credit_note_tax_rounding if credit_note_for_all_remaining_amount?
+
+      credit_note.taxes_amount_cents = credit_note.precise_taxes_amount_cents.round
       credit_note.taxes_rate = taxes_result.taxes_rate
 
       taxes_result.applied_taxes.each { |applied_tax| credit_note.applied_taxes << applied_tax }
     end
+
+    def credit_note_for_all_remaining_amount?
+      credit_note.invoice.creditable_amount_cents == 0
+    end
+
+    def adjust_credit_note_tax_rounding
+      credit_note.precise_taxes_amount_cents -= all_rounding_tax_adjustments
+    end
+
+    def all_rounding_tax_adjustments
+      credit_note.invoice.credit_notes.sum(&:taxes_rounding_adjustment)
+    end
   end
 end

app/services/credit_notes/estimate_service.rb

@@ -58,6 +58,10 @@ def validate_items
       end
     end
 
+    def valid_credit_note?
+      CreditNotes::ValidateService.new(result, item: credit_note).valid?
+    end
+
     def valid_item?(item)
       CreditNotes::ValidateItemService.new(result, item:).valid?
     end
@@ -71,21 +75,35 @@ def compute_amounts_and_taxes
       credit_note.precise_coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents
       credit_note.coupons_adjustment_amount_cents = taxes_result.coupons_adjustment_amount_cents.round
       credit_note.precise_taxes_amount_cents = taxes_result.taxes_amount_cents
-      credit_note.taxes_amount_cents = taxes_result.taxes_amount_cents.round
+      adjust_credit_note_tax_precise_rounding if credit_note_for_all_remaining_amount?
+
+      credit_note.taxes_amount_cents = credit_note.precise_taxes_amount_cents.round
       credit_note.taxes_rate = taxes_result.taxes_rate
 
       taxes_result.applied_taxes.each { |applied_tax| credit_note.applied_taxes << applied_tax }
 
       credit_note.credit_amount_cents = (
         credit_note.items.sum(&:amount_cents) -
         taxes_result.coupons_adjustment_amount_cents +
-        taxes_result.taxes_amount_cents
+        credit_note.precise_taxes_amount_cents
       ).round
 
       compute_refundable_amount
       credit_note.total_amount_cents = credit_note.credit_amount_cents
     end
 
+    def credit_note_for_all_remaining_amount?
+      credit_note.items.sum(&:precise_amount_cents) == credit_note.invoice.fees.sum(&:creditable_amount_cents)
+    end
+
+    def adjust_credit_note_tax_precise_rounding
+      credit_note.precise_taxes_amount_cents -= all_rounding_tax_adjustments
+    end
+
+    def all_rounding_tax_adjustments
+      credit_note.invoice.credit_notes.sum(&:taxes_rounding_adjustment)
+    end
+
     def compute_refundable_amount
       credit_note.refund_amount_cents = credit_note.credit_amount_cents
 

app/services/credit_notes/validate_item_service.rb

@@ -7,7 +7,6 @@ def valid?
 
       valid_item_amount?
       valid_individual_amount?
-      valid_global_amount?
 
       if errors?
         result.validation_failure!(errors:)

spec/models/credit_note_spec.rb

@@ -3,7 +3,12 @@
 require 'rails_helper'
 
 RSpec.describe CreditNote, type: :model do
-  subject(:credit_note) { create(:credit_note) }
+  subject(:credit_note) do
+    create :credit_note, credit_amount_cents: 11000, total_amount_cents: 11000, taxes_amount_cents: 1000,
+      taxes_rate: 10.0, precise_taxes_amount_cents: 1000
+  end
+
+  let(:item) { create(:credit_note_item, credit_note:, precise_amount_cents: 10000, amount_cents: 1000) }
 
   it_behaves_like 'paper_trail traceable'
 
@@ -243,10 +248,35 @@
     end
   end
 
-  describe ' #sub_total_excluding_taxes_amount_cents' do
-    it 'returs the total amount without the taxes' do
-      expect(credit_note.sub_total_excluding_taxes_amount_cents)
-        .to eq(credit_note.items.sum(&:precise_amount_cents) - credit_note.precise_coupons_adjustment_amount_cents)
+  context 'when calculating depends on related items' do
+    before do
+      item
+      credit_note.reload
+    end
+
+    describe '#sub_total_excluding_taxes_amount_cents' do
+      it 'returs the total amount without the taxes' do
+        expect(credit_note.sub_total_excluding_taxes_amount_cents)
+          .to eq(credit_note.items.sum(&:precise_amount_cents) - credit_note.precise_coupons_adjustment_amount_cents)
+      end
+    end
+
+    describe '#precise_total' do
+      it 'returns the total precise amount including precise taxes' do
+        expect(credit_note.precise_total).to eq(11000)
+      end
+    end
+  end
+
+  describe '#taxes_rounding_adjustment' do
+    it 'returns the difference between taxes and precise taxes' do
+      expect(credit_note.taxes_rounding_adjustment).to eq(0)
+    end
+  end
+
+  describe '#rounding_adjustment' do
+    it 'returns the difference between credit note total and credit note precise total' do
+      expect(credit_note.taxes_rounding_adjustment).to eq(0)
     end
   end
 
@@ -328,4 +358,36 @@
       end
     end
   end
+
+  context 'when taxes are not precise' do
+    subject(:credit_note) do
+      create :credit_note, credit_amount_cents: 8200, total_amount_cents: 8200, taxes_amount_cents: 1367,
+        taxes_rate: 20.0, precise_taxes_amount_cents: 1366.6
+    end
+
+    let(:item) { create(:credit_note_item, credit_note:, precise_amount_cents: 6833, amount_cents: 6833) }
+
+    before do
+      item
+      credit_note.reload
+    end
+
+    describe '#precise_total' do
+      it 'returns the total precise amount including precise taxes' do
+        expect(credit_note.precise_total).to eq(8199.6)
+      end
+    end
+
+    describe '#taxes_rounding_adjustment' do
+      it 'returns the difference between taxes and precise taxes' do
+        expect(credit_note.taxes_rounding_adjustment).to eq(0.4)
+      end
+    end
+
+    describe '#rounding_adjustment' do
+      it 'returns the difference between credit note total and credit note precise total' do
+        expect(credit_note.taxes_rounding_adjustment).to eq(0.4)
+      end
+    end
+  end
 end