fix: Calculate taxes if tax is based on item quantity and inclusive on item price
diff --git a/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py b/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
index 964566a..9660c95 100644
--- a/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
+++ b/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py
@@ -206,10 +206,19 @@
 			"rate": 14,
 			'included_in_print_rate': 1
 		})
+		si.append("taxes", {
+			"charge_type": "On Item Quantity",
+			"account_head": "_Test Account Education Cess - _TC",
+			"cost_center": "_Test Cost Center - _TC",
+			"description": "CESS",
+			"rate": 5,
+			'included_in_print_rate': 1
+		})
 		si.insert()
 
 		# with inclusive tax
-		self.assertEqual(si.net_total, 4385.96)
+		self.assertEqual(si.items[0].net_amount, 3947.368421052631)
+		self.assertEqual(si.net_total, 3947.37)
 		self.assertEqual(si.grand_total, 5000)
 
 		si.reload()
@@ -222,8 +231,8 @@
 		si.save()
 
 		# with inclusive tax and additional discount
-		self.assertEqual(si.net_total, 4285.96)
-		self.assertEqual(si.grand_total, 4885.99)
+		self.assertEqual(si.net_total, 3847.37)
+		self.assertEqual(si.grand_total, 4886)
 
 		si.reload()
 
@@ -235,7 +244,7 @@
 		si.save()
 
 		# with inclusive tax and additional discount
-		self.assertEqual(si.net_total, 4298.25)
+		self.assertEqual(si.net_total, 3859.65)
 		self.assertEqual(si.grand_total, 4900.00)
 
 	def test_sales_invoice_discount_amount(self):
diff --git a/erpnext/controllers/accounts_controller.py b/erpnext/controllers/accounts_controller.py
index 66b5f30..3091193 100644
--- a/erpnext/controllers/accounts_controller.py
+++ b/erpnext/controllers/accounts_controller.py
@@ -985,7 +985,7 @@
 			# all rows about the reffered tax should be inclusive
 			_on_previous_row_error("1 - %d" % (tax.row_id,))
 		elif tax.get("category") == "Valuation":
-			frappe.throw(_("Valuation type charges can not marked as Inclusive"))
+			frappe.throw(_("Valuation type charges can not be marked as Inclusive"))
 
 
 def set_balance_in_account_currency(gl_dict, account_currency=None, conversion_rate=None, company_currency=None):
diff --git a/erpnext/controllers/taxes_and_totals.py b/erpnext/controllers/taxes_and_totals.py
index 572e1ca..8f86dce 100644
--- a/erpnext/controllers/taxes_and_totals.py
+++ b/erpnext/controllers/taxes_and_totals.py
@@ -161,8 +161,9 @@
 		for item in self.doc.get("items"):
 			item_tax_map = self._load_item_tax_rate(item.item_tax_rate)
 			cumulated_tax_fraction = 0
+			total_inclusive_tax_amount_per_qty = 0
 			for i, tax in enumerate(self.doc.get("taxes")):
-				tax.tax_fraction_for_current_item = self.get_current_tax_fraction(tax, item_tax_map)
+				tax.tax_fraction_for_current_item, inclusive_tax_amount_per_qty = self.get_current_tax_fraction(tax, item_tax_map)
 
 				if i==0:
 					tax.grand_total_fraction_for_current_item = 1 + tax.tax_fraction_for_current_item
@@ -172,9 +173,12 @@
 						+ tax.tax_fraction_for_current_item
 
 				cumulated_tax_fraction += tax.tax_fraction_for_current_item
+				total_inclusive_tax_amount_per_qty += inclusive_tax_amount_per_qty * flt(item.stock_qty)
 
-			if cumulated_tax_fraction and not self.discount_amount_applied and item.qty:
-				item.net_amount = flt(item.amount / (1 + cumulated_tax_fraction))
+			if not self.discount_amount_applied and item.qty and (cumulated_tax_fraction or total_inclusive_tax_amount_per_qty):
+				amount = flt(item.amount) - total_inclusive_tax_amount_per_qty
+
+				item.net_amount = flt(amount / (1 + cumulated_tax_fraction))
 				item.net_rate = flt(item.net_amount / item.qty, item.precision("net_rate"))
 				item.discount_percentage = flt(item.discount_percentage,
 					item.precision("discount_percentage"))
@@ -190,6 +194,7 @@
 			from tax inclusive amount
 		"""
 		current_tax_fraction = 0
+		inclusive_tax_amount_per_qty = 0
 
 		if cint(tax.included_in_print_rate):
 			tax_rate = self._get_tax_rate(tax, item_tax_map)
@@ -204,10 +209,15 @@
 			elif tax.charge_type == "On Previous Row Total":
 				current_tax_fraction = (tax_rate / 100.0) * \
 					self.doc.get("taxes")[cint(tax.row_id) - 1].grand_total_fraction_for_current_item
+			
+			elif tax.charge_type == "On Item Quantity":
+				inclusive_tax_amount_per_qty = flt(tax_rate)
 
-		if getattr(tax, "add_deduct_tax", None):
-			current_tax_fraction *= -1.0 if (tax.add_deduct_tax == "Deduct") else 1.0
-		return current_tax_fraction
+		if getattr(tax, "add_deduct_tax", None) and tax.add_deduct_tax == "Deduct":
+			current_tax_fraction *= -1.0
+			inclusive_tax_amount_per_qty *= -1.0
+
+		return current_tax_fraction, inclusive_tax_amount_per_qty
 
 	def _get_tax_rate(self, tax, item_tax_map):
 		if tax.account_head in item_tax_map:
@@ -321,7 +331,7 @@
 			current_tax_amount = (tax_rate / 100.0) * \
 				self.doc.get("taxes")[cint(tax.row_id) - 1].grand_total_for_current_item
 		elif tax.charge_type == "On Item Quantity":
-			current_tax_amount = tax_rate * item.stock_qty
+			current_tax_amount = tax_rate * item.qty
 
 		self.set_item_wise_tax(item, tax, tax_rate, current_tax_amount)
 
@@ -472,7 +482,7 @@
 			actual_taxes_dict = {}
 
 			for tax in self.doc.get("taxes"):
-				if tax.charge_type == "Actual":
+				if tax.charge_type in ["Actual", "On Item Quantity"]:
 					tax_amount = self.get_tax_amount_if_for_valuation_or_deduction(tax.tax_amount, tax)
 					actual_taxes_dict.setdefault(tax.idx, tax_amount)
 				elif tax.row_id in actual_taxes_dict:
diff --git a/erpnext/public/js/controllers/taxes_and_totals.js b/erpnext/public/js/controllers/taxes_and_totals.js
index 405a33c..c0d2e6f 100644
--- a/erpnext/public/js/controllers/taxes_and_totals.js
+++ b/erpnext/public/js/controllers/taxes_and_totals.js
@@ -163,9 +163,11 @@
 		$.each(me.frm.doc["items"] || [], function(n, item) {
 			var item_tax_map = me._load_item_tax_rate(item.item_tax_rate);
 			var cumulated_tax_fraction = 0.0;
-
+			var total_inclusive_tax_amount_per_qty = 0;
 			$.each(me.frm.doc["taxes"] || [], function(i, tax) {
-				tax.tax_fraction_for_current_item = me.get_current_tax_fraction(tax, item_tax_map);
+				var current_tax_fraction = me.get_current_tax_fraction(tax, item_tax_map);
+				tax.tax_fraction_for_current_item = current_tax_fraction[0];
+				var inclusive_tax_amount_per_qty = current_tax_fraction[1];
 
 				if(i==0) {
 					tax.grand_total_fraction_for_current_item = 1 + tax.tax_fraction_for_current_item;
@@ -176,10 +178,12 @@
 				}
 
 				cumulated_tax_fraction += tax.tax_fraction_for_current_item;
+				total_inclusive_tax_amount_per_qty += inclusive_tax_amount_per_qty * flt(item.qty);
 			});
 
-			if(cumulated_tax_fraction && !me.discount_amount_applied) {
-				item.net_amount = flt(item.amount / (1 + cumulated_tax_fraction));
+			if(!me.discount_amount_applied && item.qty && (total_inclusive_tax_amount_per_qty || cumulated_tax_fraction)) {
+				var amount = flt(item.amount) - total_inclusive_tax_amount_per_qty;
+				item.net_amount = flt(amount / (1 + cumulated_tax_fraction));
 				item.net_rate = item.qty ? flt(item.net_amount / item.qty, precision("net_rate", item)) : 0;
 
 				me.set_in_company_currency(item, ["net_rate", "net_amount"]);
@@ -191,6 +195,7 @@
 		// Get tax fraction for calculating tax exclusive amount
 		// from tax inclusive amount
 		var current_tax_fraction = 0.0;
+		var inclusive_tax_amount_per_qty = 0;
 
 		if(cint(tax.included_in_print_rate)) {
 			var tax_rate = this._get_tax_rate(tax, item_tax_map);
@@ -205,13 +210,16 @@
 			} else if(tax.charge_type == "On Previous Row Total") {
 				current_tax_fraction = (tax_rate / 100.0) *
 					this.frm.doc["taxes"][cint(tax.row_id) - 1].grand_total_fraction_for_current_item;
+			} else if (tax.charge_type == "On Item Quantity") {
+				inclusive_tax_amount_per_qty = flt(tax_rate);
 			}
 		}
 
-		if(tax.add_deduct_tax) {
-			current_tax_fraction *= (tax.add_deduct_tax == "Deduct") ? -1.0 : 1.0;
+		if(tax.add_deduct_tax && tax.add_deduct_tax == "Deduct") {
+			current_tax_fraction *= -1;
+			inclusive_tax_amount_per_qty *= -1
 		}
-		return current_tax_fraction;
+		return [current_tax_fraction, inclusive_tax_amount_per_qty];
 	},
 
 	_get_tax_rate: function(tax, item_tax_map) {
@@ -360,8 +368,9 @@
 		} else if(tax.charge_type == "On Previous Row Total") {
 			current_tax_amount = (tax_rate / 100.0) *
 				this.frm.doc["taxes"][cint(tax.row_id) - 1].grand_total_for_current_item;
+		} else if (tax.charge_type == "On Item Quantity") {
+			current_tax_amount = tax_rate * item.qty;
 		}
-
 		this.set_item_wise_tax(item, tax, tax_rate, current_tax_amount);
 
 		return current_tax_amount;
@@ -573,7 +582,7 @@
 			var actual_taxes_dict = {};
 
 			$.each(this.frm.doc["taxes"] || [], function(i, tax) {
-				if (tax.charge_type == "Actual") {
+				if (in_list(["Actual", "On Item Quantity"], tax.charge_type)) {
 					var tax_amount = (tax.category == "Valuation") ? 0.0 : tax.tax_amount;
 					tax_amount *= (tax.add_deduct_tax == "Deduct") ? -1.0 : 1.0;
 					actual_taxes_dict[tax.idx] = tax_amount;
@@ -586,7 +595,7 @@
 			$.each(actual_taxes_dict, function(key, value) {
 				if (value) total_actual_tax += value;
 			});
-
+			
 			return flt(this.frm.doc.grand_total - total_actual_tax, precision("grand_total"));
 		}
 	},