feat: apply customer withholding automatically on new receipts

Tick Apply Tax Withholding Amount on unsaved customer receipts whose
references include a Sales Invoice carrying withholding: in the
get_payment_entry override (Create > Payment), in the
allocate_amount_to_references doc method (Get Outstanding Invoices /
Paid Amount changes, so the tax row updates live) and on first save.
Saved entries keep the user's choice; unticking now removes the row.
Client script recomputes the row when the checkbox or category changes.

default_thai_company/tax_withholding.py

@@ -1,6 +1,7 @@
 import erpnext
 import frappe
 from erpnext.accounts.doctype.payment_entry.payment_entry import PaymentEntry
+from erpnext.accounts.doctype.payment_entry.payment_entry import get_payment_entry as _get_payment_entry
 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,
@@ -171,6 +172,46 @@ class ThaiPaymentEntry(PaymentEntry):
 					entry.post_net_value = True
 		return gl_entries
 
+	@frappe.whitelist()
+	def allocate_amount_to_references(self, *args, **kwargs):
+		"""Called by the form after Get Outstanding Invoices and on Paid Amount
+		changes; keep the withholding row in step with the allocation."""
+		super().allocate_amount_to_references(*args, **kwargs)
+		if self.party_type == "Customer" and self.payment_type == "Receive" and self.source_exchange_rate:
+			auto_apply_customer_withholding(self)
+			set_customer_withholding(self)
+
+
+@frappe.whitelist()
+def get_payment_entry(*args, **kwargs):
+	"""Create > Payment from a Sales Invoice: arrive with withholding applied."""
+	pe = _get_payment_entry(*args, **kwargs)
+	if auto_apply_customer_withholding(pe) and pe.source_exchange_rate:
+		set_customer_withholding(pe)
+	return pe
+
+
+def auto_apply_customer_withholding(doc):
+	"""Tick Apply Tax Withholding Amount on a new customer receipt whose
+	references include a Sales Invoice that carries withholding. Only for
+	unsaved entries: after the first save the checkbox is the user's."""
+	if (
+		doc.party_type != "Customer"
+		or doc.payment_type != "Receive"
+		or doc.apply_tax_withholding_amount
+		or not doc.is_new()
+	):
+		return False
+
+	invoices = [r.reference_name for r in doc.references if r.reference_doctype == "Sales Invoice"]
+	if not invoices or not frappe.db.exists(
+		"Sales Invoice", {"name": ("in", invoices), "withholding_tax_amount": (">", 0)}
+	):
+		return False
+
+	doc.apply_tax_withholding_amount = 1
+	return True
+
 
 def set_customer_withholding(doc, method=None):
 	"""Payment Entry.validate: tax withheld by the customer on a Receive entry
@@ -184,21 +225,32 @@ def set_customer_withholding(doc, method=None):
 	the controller's validate, then re-runs the tax computation so the row's
 	base amounts and totals are final within this save.
 	"""
-	if doc.party_type != "Customer" or doc.payment_type != "Receive" or not doc.apply_tax_withholding_amount:
+	if doc.party_type != "Customer" or doc.payment_type != "Receive":
 		return
 
-	if not doc.tax_withholding_category:
-		doc.tax_withholding_category = frappe.db.get_value("Customer", doc.party, "tax_withholding_category")
+	if method == "validate":
+		auto_apply_customer_withholding(doc)
 
 	account = frappe.db.get_value(
 		"Account",
 		{"company": doc.company, "account_name": ASSET_ACCOUNT, "root_type": "Asset", "is_group": 0},
 	)
+	row = next((d for d in doc.taxes if d.account_head == account), None) if account else None
+
+	if not doc.apply_tax_withholding_amount:
+		if row:
+			doc.remove(row)
+			doc.apply_taxes()
+			doc.set_amounts_after_tax()
+		return
+
 	if not account:
 		frappe.throw(_("Account {0} not found for Company {1}").format(ASSET_ACCOUNT, doc.company))
 
+	if not doc.tax_withholding_category:
+		doc.tax_withholding_category = frappe.db.get_value("Customer", doc.party, "tax_withholding_category")
+
 	amount, description = get_customer_withholding(doc)
-	row = next((d for d in doc.taxes if d.account_head == account), None)
 
 	if not amount:
 		if row: