fix: multiple rounding issue (backport #4953)

[mergify]

---

This is an automatic backport of pull request #4953 done by [Mergify](https://mergify.com).

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics -6 complexity

Metric	Results
Complexity	-6

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[greptile-apps]

Confidence Score: 4/5

[Medium risk] Fixes rounding precision in tax calculations across multiple modules.

Do not merge until advance-tax calculations and ledger posting use consistent company-currency precision.

Findings

1.  Advance GST stays partly unreversed ▶

_{Reviews (1) · Last reviewed commit: "fix: use field precision for advance gst..." · Reviewed by Greptile}

[greptile-apps]

Advance GST stays partly unreversed

If currency_precision is unset and currency-specific number formats are enabled, get_proportionate_tax chooses precision without the payment document or company currency. This can use the site's decimal count instead of the company's, while ERPNext rounds ledger amounts to the company currency.

For an INR company with two decimals on a site with three, separately allocating ₹33.30, ₹33.30, and ₹933.40 against a ₹1,000 advance with ₹180 GST calculates reversal shares of ₹5.994, ₹5.994, and ₹168.012. Ledger rounding reduces those to ₹5.99, ₹5.99, and ₹168.01, reversing only ₹179.99. The final allocation enters balance_taxes, but subtracts the earlier three-decimal shares rather than their rounded ledger amounts, so it cannot recover the missing paisa.

Use company-currency precision for proportional calculations and final balancing, and cover separate allocations with different site and currency formats in a regression test. This financial accuracy issue must be fixed before merging.

Artifacts

Complete executable GST precision check

• The authored script extracts and executes real base/head functions and snapshot/version-16 contracts with explicit service stubs, making the focused reproduction inspectable.

Base execution with full GST reversal

• Executed the three separate allocations against base with both contract sets and captured all extracted source and output, showing ₹180.00 reversed with no residual.

Head execution with one paisa left unreversed

• Executed the identical allocations against head with both contract sets and captured the final balancing and GL rounding output, showing ₹179.99 reversed and ₹0.01 remaining.

_{Ran code and verified through T-Rex}