[ahmedm10]

Ticket

https://miceportal.atlassian.net/browse/MP-54008

Story

Auftragsbuch's invoice rows are gross-leading: the gross price is fixed and net and VAT are derived from it. That VAT can be one cent off net × rate — 8 nights at 52.00 gross give net 388.79 and VAT 27.21, while 388.79 × 7 % rounds to 27.22. The gem demands exact equality, so Auftragsbuch has to let it recalculate the VAT, and the XML then says 27.22 / 416.01 while the PDF says 27.21 / 416.00. The XML is the legally relevant part of a ZUGFeRD invoice, so it must carry the invoice's own amounts. EN 16931 allows this rounding itself (BR-CO-17).

With this change a line's VAT may differ from net × rate by at most one cent, and the :ITEM_BASED check — which Auftragsbuch will use to pass the rows' own VAT — actually works: it compared a string with a number, only recorded the mismatch and still reported the invoice as valid.

The second commit fixes the same drift on the invoice level. The gem summed the VAT base (BT-116) from unit price × quantity instead of the line totals. When unit prices are stored with four decimals the two drift apart: 2 days × 120 participants at 41.6667 (5,000.00 net per day) gave lines of 5,000.00 + 5,000.00 but a VAT base and total of 10,000.01 — an XML that contradicts itself (BR-CO-10, BR-S-08). The base is now the sum of the line totals, as EN 16931 defines it.

How to review

Two commits, best read one by one:

1. fix: allow 1-cent line VAT rounding
   • lib/secretariat/line_item.rb — LINE_TAX_ROUNDING_TOLERANCE and the one changed comparison in #valid?.
   • lib/secretariat/invoice.rb — the :ITEM_BASED branch of #valid?.
   • test/invoice_test.rb — four tests: the gross-leading invoice is accepted and its XML carries 27.21 / 416.00; a line two cents off is still refused; item-based totals passed as strings leave no error; an item-based total that differs from its lines is refused.
2. fix: use line totals as the VAT base
   • lib/secretariat/line_item.rb — #signed_charge_amount.
   • lib/secretariat/invoice.rb — #taxes sums it instead of unit price × quantity.
   • test/invoice_test.rb — the two-day example: VAT base 10000.00, not 10000.01.

All tests were written before the change.

Design Choices

• Tolerance on the line, exact everywhere else. The invoice-level checks (tax total, basis, grand total) stay exact. EN 16931 has no line-level VAT amount — the line VAT only feeds the VAT breakdown — so one cent of rounding there is the smallest relaxation that lets the invoice carry its own figures.
• Branched from upstream (halfbyte) main, not from our main. The same commit is offered upstream as well; upstream has related rounding PRs open since January (Use charge_amount for tax base calculation instead of multiplying net amount with quantity halfbyte/ruby-secretariat#45, Rounding issue when summing tax amount halfbyte/ruby-secretariat#51) and no merges since April, so we do not wait for it — this PR is the one we use.
• VAT base from the line totals, with their sign. Same idea as Use charge_amount for tax base calculation instead of multiplying net amount with quantity halfbyte/ruby-secretariat#45, but a negative quantity keeps its negative line total — the rule the gem already uses when it checks that the lines add up to the basis. Use charge_amount for tax base calculation instead of multiplying net amount with quantity halfbyte/ruby-secretariat#45's other changes (contact fields, to_xml side effects) are not taken: the contact conflicts with our fork's own, the rest is unrelated to this ticket.
• Alternative rejected: to_xml(validate: false) would switch off all of the gem's checks instead of relaxing one.

Links

• Upstream PR: Allow 1-cent rounding on line VAT, use line totals as the VAT base, fix the ITEM_BASED check halfbyte/ruby-secretariat#62
• Previous step of this ticket: MP-54008 Sync with upstream v3.9.0 #6 (sync with upstream v3.9.0)
• Related upstream PRs: Use charge_amount for tax base calculation instead of multiplying net amount with quantity halfbyte/ruby-secretariat#45, Rounding issue when summing tax amount halfbyte/ruby-secretariat#51

Checklist

• I self-reviewed the PR
• I got approval from the design team
• I checked the acceptance criteria
• I added the PR description