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/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."""
-		if is_thai_company(self.company):
+		the invoice. For Thai companies only show the expected withholding and
+		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(
-			ref.reference_doctype, ref.reference_name, ["net_total", "grand_total"]
-		)
-		if not grand_total:
+		fields = ["net_total", "grand_total"]
+		if ref.reference_doctype == "Sales Invoice":
+			fields.append("tax_withholding_category")
+		values = frappe.db.get_value(ref.reference_doctype, ref.reference_name, fields, as_dict=True)
+		if not values.grand_total:
 			continue
 
-		taxable = (
-			flt(ref.allocated_amount) * flt(net_total) / flt(grand_total) * flt(doc.source_exchange_rate)
-		)
-		if details.threshold and taxable < details.threshold:
-			continue
-		total += taxable * flt(details.rate) / 100
+		category = values.get("tax_withholding_category") or doc.tax_withholding_category
+		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 category not in details_by_category:
+			details_by_category[category] = get_withholding_details(category, doc.posting_date, doc.company)
 
-	if cint(details.round_off_tax_amount):
-		return normal_round(total)
-	return flt(total, doc.precision("difference_amount"))
+		base_taxable = (
+			flt(ref.allocated_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