chore: Tests

- Added test for mapping secnd return doc
- Added test for billing % of partially returned doc
- Handled PR with 0 billing amount
diff --git a/erpnext/stock/doctype/delivery_note/test_delivery_note.py b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
index 9f273d7..fa07a25 100644
--- a/erpnext/stock/doctype/delivery_note/test_delivery_note.py
+++ b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
@@ -255,6 +255,32 @@
 		self.assertEqual(dn.items[0].returned_qty, 2)
 		self.assertEqual(dn.per_returned, 40)
 
+		from erpnext.controllers.sales_and_purchase_return import make_return_doc
+		return_dn_2 = make_return_doc("Delivery Note", dn.name)
+
+		# Check if unreturned amount is mapped in 2nd return
+		self.assertEqual(return_dn_2.items[0].qty, -3)
+
+		si = make_sales_invoice(dn.name)
+		si.submit()
+
+		self.assertEqual(si.items[0].qty, 3)
+
+		dn.load_from_db()
+		# DN should be completed on billing all unreturned amount
+		self.assertEqual(dn.items[0].billed_amt, 1500)
+		self.assertEqual(dn.per_billed, 100)
+		self.assertEqual(dn.status, 'Completed')
+
+		si.load_from_db()
+		si.cancel()
+
+		dn.load_from_db()
+		self.assertEqual(dn.per_billed, 0)
+
+		dn1.cancel()
+		dn.cancel()
+
 	def test_sales_return_for_non_bundled_items_full(self):
 		from erpnext.stock.doctype.item.test_item import make_item
 
diff --git a/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py b/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py
index c37740c..1852985 100644
--- a/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py
+++ b/erpnext/stock/doctype/purchase_receipt/purchase_receipt.py
@@ -513,7 +513,7 @@
 		where po_detail=%s and (pr_detail is null or pr_detail = '') and docstatus=1""", po_detail)
 	billed_against_po = billed_against_po and billed_against_po[0][0] or 0
 
-	# Get all Delivery Note Item rows against the Sales Order Item row
+	# Get all Purchase Receipt Item rows against the Purchase Order Item row
 	pr_details = frappe.db.sql("""select pr_item.name, pr_item.amount, pr_item.parent
 		from `tabPurchase Receipt Item` pr_item, `tabPurchase Receipt` pr
 		where pr.name=pr_item.parent and pr_item.purchase_order_item=%s
@@ -544,6 +544,9 @@
 	return updated_pr
 
 def update_billing_percentage(pr_doc, update_modified=True):
+	# Reload as billed amount was set in db directly
+	pr_doc.load_from_db()
+
 	# Update Billing % based on pending accepted qty
 	total_amount, total_billed_amount = 0, 0
 	for item in pr_doc.items:
@@ -567,7 +570,7 @@
 		total_amount += total_billable_amount
 		total_billed_amount += flt(item.billed_amt)
 
-	percent_billed = round(100 * (total_billed_amount / total_amount), 6)
+	percent_billed = round(100 * (total_billed_amount / (total_amount or 1)), 6)
 	pr_doc.db_set("per_billed", percent_billed)
 	pr_doc.load_from_db()
 
diff --git a/erpnext/stock/doctype/purchase_receipt/test_purchase_receipt.py b/erpnext/stock/doctype/purchase_receipt/test_purchase_receipt.py
index aef5bf3..c23d6c2 100644
--- a/erpnext/stock/doctype/purchase_receipt/test_purchase_receipt.py
+++ b/erpnext/stock/doctype/purchase_receipt/test_purchase_receipt.py
@@ -100,7 +100,10 @@
 		self.assertFalse(frappe.db.get_all('Serial No', {'batch_no': batch_no}))
 
 	def test_purchase_receipt_gl_entry(self):
-		pr = make_purchase_receipt(company="_Test Company with perpetual inventory", warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1", get_multiple_items = True, get_taxes_and_charges = True)
+		pr = make_purchase_receipt(company="_Test Company with perpetual inventory",
+			warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1",
+			get_multiple_items = True, get_taxes_and_charges = True)
+
 		self.assertEqual(cint(erpnext.is_perpetual_inventory_enabled(pr.company)), 1)
 
 		gl_entries = get_gl_entries("Purchase Receipt", pr.name)
@@ -245,10 +248,12 @@
 				pr.get("items")[0].rejected_warehouse)
 
 	def test_purchase_return_partial(self):
+		pr = make_purchase_receipt(company="_Test Company with perpetual inventory",
+			warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1")
 
-		pr = make_purchase_receipt(company="_Test Company with perpetual inventory", warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1")
-
-		return_pr = make_purchase_receipt(company="_Test Company with perpetual inventory", warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1", is_return=1, return_against=pr.name, qty=-2, do_not_submit=1)
+		return_pr = make_purchase_receipt(company="_Test Company with perpetual inventory",
+			warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1",
+			is_return=1, return_against=pr.name, qty=-2, do_not_submit=1)
 		return_pr.items[0].purchase_receipt_item = pr.items[0].name
 		return_pr.submit()
 
@@ -283,6 +288,33 @@
 		self.assertEqual(pr.items[0].returned_qty, 2)
 		self.assertEqual(pr.per_returned, 40)
 
+		from erpnext.controllers.sales_and_purchase_return import make_return_doc
+		return_pr_2 = make_return_doc("Purchase Receipt", pr.name)
+
+		# Check if unreturned amount is mapped in 2nd return
+		self.assertEqual(return_pr_2.items[0].qty, -3)
+
+		# Make PI against unreturned amount
+		pi = make_purchase_invoice(pr.name)
+		pi.submit()
+
+		self.assertEqual(pi.items[0].qty, 3)
+
+		pr.load_from_db()
+		# PR should be completed on billing all unreturned amount
+		self.assertEqual(pr.items[0].billed_amt, 150)
+		self.assertEqual(pr.per_billed, 100)
+		self.assertEqual(pr.status, 'Completed')
+
+		pi.load_from_db()
+		pi.cancel()
+
+		pr.load_from_db()
+		self.assertEqual(pr.per_billed, 0)
+
+		return_pr.cancel()
+		pr.cancel()
+
 	def test_purchase_return_full(self):
 		pr = make_purchase_receipt(company="_Test Company with perpetual inventory", warehouse = "Stores - TCP1", supplier_warehouse = "Work in Progress - TCP1")
 
@@ -406,6 +438,7 @@
 		self.assertEqual(pr1.per_billed, 100)
 		self.assertEqual(pr1.status, "Completed")
 
+		pr2.load_from_db()
 		self.assertEqual(pr2.get("items")[0].billed_amt, 2000)
 		self.assertEqual(pr2.per_billed, 80)
 		self.assertEqual(pr2.status, "To Bill")