diff --git a/addons/account/models/account.py b/addons/account/models/account.py
index 91a95ba359d..1d1d50cfc01 100644
--- a/addons/account/models/account.py
+++ b/addons/account/models/account.py
@@ -615,7 +615,16 @@ class AccountTax(models.Model):
         if self.amount_type == 'fixed':
             # Use copysign to take into account the sign of the base amount which includes the sign
             # of the quantity and the sign of the price_unit
-            return math.copysign(quantity, base_amount) * self.amount
+            # Amount is the fixed price for the tax, it can be negative
+            # Base amount included the sign of the quantity and the sign of the unit price and when
+            # a product is returned, it can be done either by changing the sign of quantity or by changing the
+            # sign of the price unit.
+            # When the price unit is equal to 0, the sign of the quantity is absorbed in base_amount then
+            # a "else" case is needed.
+            if base_amount:
+                return math.copysign(quantity, base_amount) * self.amount
+            else:
+                return quantity * self.amount
         if (self.amount_type == 'percent' and not self.price_include) or (self.amount_type == 'division' and self.price_include):
             return base_amount * self.amount / 100
         if self.amount_type == 'percent' and self.price_include: