feat: show expected withholding tax on Sales Invoice

Custom fields Tax Withholding Category (fetched from Customer),
Withholding Tax and Net Payable After Withholding Tax on Sales Invoice;
computed in the Thai set_tax_withholding override from the category
rate and single threshold, informational only (totals/GL unchanged),
printed under Rounded Total by the standard layout. Payment Entry now
prefers each invoice's category over the customer's.

default_thai_company/fixtures/custom_field.json

@@ -0,0 +1,50 @@
+[
+ {
+  "docstatus": 0,
+  "doctype": "Custom Field",
+  "dt": "Sales Invoice",
+  "fetch_from": "customer.tax_withholding_category",
+  "fetch_if_empty": 1,
+  "fieldname": "tax_withholding_category",
+  "fieldtype": "Link",
+  "insert_after": "taxes_and_charges",
+  "label": "Tax Withholding Category",
+  "modified": "2026-09-11 18:00:00.000000",
+  "name": "Sales Invoice-tax_withholding_category",
+  "options": "Tax Withholding Category",
+  "print_hide": 1
+ },
+ {
+  "depends_on": "eval:doc.tax_withholding_category",
+  "docstatus": 0,
+  "doctype": "Custom Field",
+  "dt": "Sales Invoice",
+  "fieldname": "withholding_tax_amount",
+  "fieldtype": "Currency",
+  "insert_after": "rounded_total",
+  "label": "Withholding Tax",
+  "modified": "2026-09-11 18:00:00.000000",
+  "name": "Sales Invoice-withholding_tax_amount",
+  "no_copy": 1,
+  "options": "currency",
+  "print_hide_if_no_value": 1,
+  "read_only": 1
+ },
+ {
+  "bold": 1,
+  "depends_on": "eval:doc.tax_withholding_category",
+  "docstatus": 0,
+  "doctype": "Custom Field",
+  "dt": "Sales Invoice",
+  "fieldname": "amount_after_withholding",
+  "fieldtype": "Currency",
+  "insert_after": "withholding_tax_amount",
+  "label": "Net Payable After Withholding Tax",
+  "modified": "2026-09-11 18:00:00.000000",
+  "name": "Sales Invoice-amount_after_withholding",
+  "no_copy": 1,
+  "options": "currency",
+  "print_hide_if_no_value": 1,
+  "read_only": 1
+ }
+]

default_thai_company/hooks.py

@@ -102,6 +102,20 @@ fixtures = [
 		],
 	},
 	{"doctype": "Tax Withholding Category", "filters": [["name", "like", "WHT %"]]},
+	{
+		"doctype": "Custom Field",
+		"filters": [
+			[
+				"name",
+				"in",
+				[
+					"Sales Invoice-tax_withholding_category",
+					"Sales Invoice-withholding_tax_amount",
+					"Sales Invoice-amount_after_withholding",
+				],
+			]
+		],
+	},
 ]
 
 # Installation

default_thai_company/tax_withholding.py

@@ -116,14 +116,45 @@ def is_thai_company(company):
 	return frappe.get_cached_value("Company", company, "country") == "Thailand"
 
 
+def get_withholding_details(category, posting_date, company):
+	details = get_tax_withholding_details(category, posting_date, company)
+	if not details:
+		frappe.throw(
+			_("Tax Withholding Category {0} has no account for Company {1}").format(category, company)
+		)
+	return details
+
+
+def withholding_on(details, taxable, base_taxable, precision):
+	"""Withholding for a taxable amount; the single threshold is checked in company currency."""
+	if details.threshold and flt(base_taxable) < flt(details.threshold):
+		return 0.0
+	amount = flt(taxable) * flt(details.rate) / 100
+	return normal_round(amount) if cint(details.round_off_tax_amount) else flt(amount, precision)
+
+
 class ThaiSalesInvoice(SalesInvoice):
 	def set_tax_withholding(self):
 		"""Thai customers withhold at payment (see `set_customer_withholding`);
 		ERPNext's customer-side handling is Indian TCS, which adds tax on top of
-		the invoice. Skip it for Thai companies."""
+		the invoice. For Thai companies only show the expected withholding and
-		if is_thai_company(self.company):
+		the net payable; totals and GL are untouched."""
+		if not is_thai_company(self.company):
+			return super().set_tax_withholding()
+
+		self.withholding_tax_amount = self.amount_after_withholding = 0
+		if not self.tax_withholding_category:
 			return
-		super().set_tax_withholding()
+
+		details = get_withholding_details(self.tax_withholding_category, self.posting_date, self.company)
+		self.withholding_tax_amount = withholding_on(
+			details, self.net_total, self.base_net_total, self.precision("withholding_tax_amount")
+		)
+		if self.withholding_tax_amount:
+			self.amount_after_withholding = flt(
+				(self.rounded_total or self.grand_total) - self.withholding_tax_amount,
+				self.precision("amount_after_withholding"),
+			)
 
 
 def set_customer_withholding(doc, method=None):
@@ -141,16 +172,6 @@ def set_customer_withholding(doc, method=None):
 
 	if not doc.tax_withholding_category:
 		doc.tax_withholding_category = frappe.db.get_value("Customer", doc.party, "tax_withholding_category")
-	if not doc.tax_withholding_category:
-		frappe.throw(_("Please set Tax Withholding Category for Customer {0}").format(doc.party))
-
-	details = get_tax_withholding_details(doc.tax_withholding_category, doc.posting_date, doc.company)
-	if not details:
-		frappe.throw(
-			_("Tax Withholding Category {0} has no account for Company {1}").format(
-				doc.tax_withholding_category, doc.company
-			)
-		)
 
 	account = frappe.db.get_value(
 		"Account",
@@ -159,7 +180,7 @@ def set_customer_withholding(doc, method=None):
 	if not account:
 		frappe.throw(_("Account {0} not found for Company {1}").format(ASSET_ACCOUNT, doc.company))
 
-	amount = get_customer_withholding_amount(doc, details)
+	amount, description = get_customer_withholding(doc)
 	row = next((d for d in doc.deductions if d.account == account), None)
 
 	if not amount:
@@ -169,32 +190,55 @@ def set_customer_withholding(doc, method=None):
 		if not row:
 			row = doc.append("deductions", {"account": account})
 		row.amount = amount
-		row.description = details.description
+		row.description = description
 		row.cost_center = doc.cost_center or erpnext.get_default_cost_center(doc.company)
 
 	doc.set_unallocated_amount()
 	doc.set_difference_amount()
 
 
-def get_customer_withholding_amount(doc, details):
+def get_customer_withholding(doc):
+	"""Total withheld across allocated references, in company currency.
+
+	A Sales Invoice's own category wins over the Payment Entry's; Sales Orders
+	use the Payment Entry's. Withholding is prorated by allocated / grand total
+	on the pre-VAT amount, per reference.
+	"""
 	total = 0.0
+	details_by_category = {}
+	precision = doc.precision("difference_amount")
+
 	for ref in doc.references:
 		if ref.reference_doctype not in ("Sales Invoice", "Sales Order") or not ref.allocated_amount:
 			continue
 
-		net_total, grand_total = frappe.db.get_value(
+		fields = ["net_total", "grand_total"]
-			ref.reference_doctype, ref.reference_name, ["net_total", "grand_total"]
+		if ref.reference_doctype == "Sales Invoice":
-		)
+			fields.append("tax_withholding_category")
-		if not grand_total:
+		values = frappe.db.get_value(ref.reference_doctype, ref.reference_name, fields, as_dict=True)
+		if not values.grand_total:
 			continue
 
-		taxable = (
+		category = values.get("tax_withholding_category") or doc.tax_withholding_category
-			flt(ref.allocated_amount) * flt(net_total) / flt(grand_total) * flt(doc.source_exchange_rate)
+		if not category:
+			frappe.throw(
+				_("Please set Tax Withholding Category on {0} {1} or on this Payment Entry").format(
+					_(ref.reference_doctype), ref.reference_name
 				)
-		if details.threshold and taxable < details.threshold:
+			)
-			continue
+		if category not in details_by_category:
-		total += taxable * flt(details.rate) / 100
+			details_by_category[category] = get_withholding_details(category, doc.posting_date, doc.company)
 
-	if cint(details.round_off_tax_amount):
+		base_taxable = (
-		return normal_round(total)
+			flt(ref.allocated_amount)
-	return flt(total, doc.precision("difference_amount"))
+			* flt(values.net_total)
+			/ flt(values.grand_total)
+			* flt(doc.source_exchange_rate)
+		)
+		total += withholding_on(details_by_category[category], base_taxable, base_taxable, precision)
+
+	if len(details_by_category) == 1:
+		description = next(iter(details_by_category.values())).description
+	else:
+		description = _("Withholding tax deducted by customer")
+	return flt(total, precision), description

default_thai_company/tests/test_tax_withholding.py

@@ -10,6 +10,7 @@ ABBR = "_TWC"
 CUSTOMER = "_Test WHT Customer"
 ITEM = "_Test WHT Service"
 FIXTURE = frappe.get_app_path("default_thai_company", "fixtures", "tax_withholding_category.json")
+CUSTOM_FIELDS = frappe.get_app_path("default_thai_company", "fixtures", "custom_field.json")
 
 
 class TestTaxWithholding(FrappeTestCase):
@@ -19,6 +20,8 @@ class TestTaxWithholding(FrappeTestCase):
 		# Fixture categories must exist before the company is created so the
 		# Company hook has something to link; force=True re-imports.
 		import_file_by_path(FIXTURE, force=True, data_import=True)
+		import_file_by_path(CUSTOM_FIELDS, force=True, data_import=True)
+		frappe.clear_cache(doctype="Sales Invoice")
 
 		frappe.get_doc(
 			{
@@ -64,7 +67,7 @@ class TestTaxWithholding(FrappeTestCase):
 			}
 		).insert()
 
-	def make_invoice(self, rate=10000):
+	def make_invoice(self, rate=10000, category=None):
 		si = frappe.get_doc(
 			{
 				"doctype": "Sales Invoice",
@@ -72,6 +75,7 @@ class TestTaxWithholding(FrappeTestCase):
 				"customer": CUSTOMER,
 				"posting_date": nowdate(),
 				"due_date": nowdate(),
+				"tax_withholding_category": category,
 				"items": [{"item_code": ITEM, "qty": 1, "rate": rate}],
 				"taxes": [
 					{
@@ -158,10 +162,24 @@ class TestTaxWithholding(FrappeTestCase):
 		rates = frappe.get_doc("Tax Withholding Category", "WHT 5% - Rent").rates
 		self.assertEqual([(r.tax_withholding_rate, r.single_threshold) for r in rates], [(5.0, 1000.0)])
 
-	def test_sales_invoice_is_not_grossed_up(self):
+	def test_sales_invoice_shows_withholding_without_grossing_up(self):
 		si = self.make_invoice()
 		self.assertEqual((si.net_total, si.grand_total), (10000.0, 10700.0))
 		self.assertEqual([t.account_head for t in si.taxes], [self.vat.name])
+		self.assertEqual(si.tax_withholding_category, "WHT 3% - Service")  # fetched from Customer
+		self.assertEqual((si.withholding_tax_amount, si.amount_after_withholding), (300.0, 10400.0))
+		self.assertEqual(frappe.db.get_value("Sales Invoice", si.name, "outstanding_amount"), 10700.0)
+
+	def test_sales_invoice_below_threshold_shows_nothing(self):
+		si = self.make_invoice(rate=800)
+		self.assertEqual((si.withholding_tax_amount, si.amount_after_withholding), (0.0, 0.0))
+
+	def test_receipt_uses_invoice_category_over_customer_category(self):
+		si = self.make_invoice(category="WHT 5% - Rent")
+		self.assertEqual(si.withholding_tax_amount, 500.0)
+		pe = self.make_receipt(si, allocated=10700, paid=10200)
+		self.assertEqual([(d.account, d.amount) for d in pe.deductions], [(self.receivable, 500.0)])
+		self.assertEqual(pe.difference_amount, 0)
 
 	def test_receipt_deducts_withholding_and_settles_invoice(self):
 		si = self.make_invoice()