feat: customer-side withholding tax on Payment Entry

Thai customers withhold tax at payment; ERPNext's customer-side handling
is Indian TCS which adds tax on top of the Sales Invoice. Override
SalesInvoice.set_tax_withholding to no-op for Thai companies, expose
"Apply Tax Withholding Amount" on Customer/Receive Payment Entries, and
on validate compute rate x pre-VAT amount of each allocated reference
(prorated, single threshold honoured) as a deduction to "Withholding Tax
Receivable". Client script fetches the Customer's category when the box
is ticked.

default_thai_company/tax_withholding.py

@@ -1,4 +1,12 @@
+import erpnext
 import frappe
+from erpnext.accounts.doctype.sales_invoice.sales_invoice import SalesInvoice
+from erpnext.accounts.doctype.tax_withholding_category.tax_withholding_category import (
+	get_tax_withholding_details,
+	normal_round,
+)
+from frappe import _
+from frappe.utils import cint, flt
 
 # Liability: tax we withhold from suppliers and remit on P.N.D.3/53 (linked on every category).
 # Asset: tax customers withhold from us, creditable against corporate income tax.
@@ -102,3 +110,91 @@ def setup_company(doc, method=None):
 		category = frappe.get_doc("Tax Withholding Category", name)
 		if link_company(category, doc.name, account):
 			category.save(ignore_permissions=True)
+
+
+def is_thai_company(company):
+	return frappe.get_cached_value("Company", company, "country") == "Thailand"
+
+
+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):
+			return
+		super().set_tax_withholding()
+
+
+def set_customer_withholding(doc, method=None):
+	"""Payment Entry.validate: tax withheld by the customer on a Receive entry
+	becomes a deduction to the company's withholding receivable account.
+
+	Withholding is rate x pre-VAT amount of each allocated reference, prorated
+	by the allocation. `paid_amount` is the cash actually received, so the
+	deduction closes the difference against the gross allocation. Runs after
+	the controller's validate (exchange rates and allocations are final), then
+	re-derives the two amounts that depend on deductions.
+	"""
+	if doc.party_type != "Customer" or doc.payment_type != "Receive" or not doc.apply_tax_withholding_amount:
+		return
+
+	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",
+		{"company": doc.company, "account_name": ASSET_ACCOUNT, "root_type": "Asset", "is_group": 0},
+	)
+	if not account:
+		frappe.throw(_("Account {0} not found for Company {1}").format(ASSET_ACCOUNT, doc.company))
+
+	amount = get_customer_withholding_amount(doc, details)
+	row = next((d for d in doc.deductions if d.account == account), None)
+
+	if not amount:
+		if row:
+			doc.remove(row)
+	else:
+		if not row:
+			row = doc.append("deductions", {"account": account})
+		row.amount = amount
+		row.description = details.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):
+	total = 0.0
+	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:
+			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
+
+	if cint(details.round_off_tax_amount):
+		return normal_round(total)
+	return flt(total, doc.precision("difference_amount"))