diff --git a/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py b/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
index de488aebcbf..9ee30acbb77 100644
--- a/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
+++ b/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
@@ -4305,6 +4305,31 @@ class TestSalesInvoice(IntegrationTestCase):
 		doc = frappe.get_doc("Project", project.name)
 		self.assertEqual(doc.total_billed_amount, si.grand_total)
 
+	def test_total_billed_amount_with_different_projects(self):
+		# This test case is for checking the scenario where project is set at document level and for **some** child items only, not all
+		from copy import copy
+
+		si = create_sales_invoice(do_not_submit=True)
+
+		project = frappe.new_doc("Project")
+		project.company = "_Test Company"
+		project.project_name = "Test Total Billed Amount"
+		project.save()
+
+		si.project = project.name
+		si.items.append(copy(si.items[0]))
+		si.items.append(copy(si.items[0]))
+		si.items[0].project = project.name
+		si.items[1].project = project.name
+		# Not setting project on last item
+		si.items[1].insert()
+		si.items[2].insert()
+		si.submit()
+
+		project.reload()
+		self.assertIsNone(si.items[2].project)
+		self.assertEqual(project.total_billed_amount, 300)
+
 	def test_pos_returns_with_party_account_currency(self):
 		from erpnext.accounts.doctype.sales_invoice.sales_invoice import make_sales_return
 
diff --git a/erpnext/projects/doctype/project/project.py b/erpnext/projects/doctype/project/project.py
index 5706edcda63..959b34f77a3 100644
--- a/erpnext/projects/doctype/project/project.py
+++ b/erpnext/projects/doctype/project/project.py
@@ -322,17 +322,31 @@ class Project(Document):
 		self.total_sales_amount = total_sales_amount and total_sales_amount[0][0] or 0
 
 	def update_billed_amount(self):
-		# nosemgrep
+		self.total_billed_amount = self.get_billed_amount_from_parent() + self.get_billed_amount_from_child()
+
+	def get_billed_amount_from_parent(self):
 		total_billed_amount = frappe.db.sql(
 			"""select sum(base_net_amount)
-			from `tabSales Invoice Item` si_item, `tabSales Invoice` si
-			where si_item.parent = si.name
-				and if(si_item.project, si_item.project, si.project) = %s
-				and si.docstatus=1""",
+			from `tabSales Invoice` si join `tabSales Invoice Item` si_item on si_item.parent = si.name
+				where si_item.project is null
+				and si.project is not null
+				and si.project = %s
+				and si.docstatus = 1""",
 			self.name,
 		)
 
-		self.total_billed_amount = total_billed_amount and total_billed_amount[0][0] or 0
+		return total_billed_amount and total_billed_amount[0][0] or 0
+
+	def get_billed_amount_from_child(self):
+		total_billed_amount = frappe.db.sql(
+			"""select sum(base_net_amount)
+			from `tabSales Invoice Item`
+				where project = %s
+				and docstatus = 1""",
+			self.name,
+		)
+
+		return total_billed_amount and total_billed_amount[0][0] or 0
 
 	def after_rename(self, old_name, new_name, merge=False):
 		if old_name == self.copied_from: