diff --git a/addons/account/models/account_tax.py b/addons/account/models/account_tax.py
index ee72a6883fa..7f16d1894a5 100644
--- a/addons/account/models/account_tax.py
+++ b/addons/account/models/account_tax.py
@@ -502,7 +502,7 @@ class AccountTax(models.Model):
                     elif tax.amount_type == 'division':
                         incl_division_amount += tax.amount * sum_repartition_factor
                     elif tax.amount_type == 'fixed':
-                        incl_fixed_amount += quantity * tax.amount * sum_repartition_factor
+                        incl_fixed_amount += abs(quantity) * tax.amount * sum_repartition_factor
                     else:
                         # tax.amount_type == other (python)
                         tax_amount = tax._compute_amount(base, sign * price_unit, quantity, product, partner) * sum_repartition_factor
diff --git a/addons/point_of_sale/static/src/js/models.js b/addons/point_of_sale/static/src/js/models.js
index 43968c916fa..b5a9b92fc19 100644
--- a/addons/point_of_sale/static/src/js/models.js
+++ b/addons/point_of_sale/static/src/js/models.js
@@ -2182,7 +2182,7 @@ exports.Orderline = Backbone.Model.extend({
                     else if(tax.amount_type === 'division')
                         incl_division_amount += tax.amount;
                     else if(tax.amount_type === 'fixed')
-                        incl_fixed_amount += quantity * tax.amount
+                        incl_fixed_amount += Math.abs(quantity) * tax.amount
                     else{
                         var tax_amount = self._compute_all(tax, base, quantity);
                         incl_fixed_amount += tax_amount;