diff --git a/modules/edocument_peppol/CHANGELOG b/modules/edocument_peppol/CHANGELOG
index 260f651100..16db0745d5 100644
--- a/modules/edocument_peppol/CHANGELOG
+++ b/modules/edocument_peppol/CHANGELOG
@@ -1,3 +1,4 @@
+* Ensure that the unit price on the invoice line is not negative
 * Add an "Update Status" button to the Peppol document
 
 Version 7.8.0 - 2025-12-15
diff --git a/modules/edocument_peppol/account.py b/modules/edocument_peppol/account.py
index 0b36299ae0..c5105f39ac 100644
--- a/modules/edocument_peppol/account.py
+++ b/modules/edocument_peppol/account.py
@@ -6,7 +6,7 @@ from trytond.model import fields
 from trytond.pool import Pool, PoolMeta
 from trytond.pyson import Eval
 
-from .exceptions import InvoicePeppolRequired
+from .exceptions import InvoicePeppolError, InvoicePeppolRequired
 
 
 class Invoice(metaclass=PoolMeta):
@@ -47,6 +47,7 @@ class Invoice(metaclass=PoolMeta):
             peppol_types = invoice.party.get_multivalue(
                 'peppol_types', company=invoice.company.id)
             if peppol_types and 'bis-billing-3' in peppol_types:
+                invoice.check_peppol()
                 peppol.append(Peppol(
                         direction='out',
                         company=invoice.company,
@@ -65,3 +66,13 @@ class Invoice(metaclass=PoolMeta):
         if peppol:
             Peppol.save(peppol)
             Peppol.submit(peppol)
+
+    def check_peppol(self):
+        "Check if the invoice is valid for Peppol"
+        for line in self.lines:
+            if line.type == 'line' and line.unit_price < 0:  # BR-27
+                raise InvoicePeppolError(gettext(
+                        'edocument_peppol.'
+                        'msg_invoice_line_unit_price_not_negative',
+                        invoice=self.rec_name,
+                        line=line.rec_name))
diff --git a/modules/edocument_peppol/exceptions.py b/modules/edocument_peppol/exceptions.py
index 049e5c83a4..69cc23946c 100644
--- a/modules/edocument_peppol/exceptions.py
+++ b/modules/edocument_peppol/exceptions.py
@@ -1,12 +1,16 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 
-from trytond.exceptions import UserWarning
+from trytond.exceptions import UserError, UserWarning
 
 
 class InvoicePeppolRequired(UserWarning):
     pass
 
 
+class InvoicePeppolError(UserError):
+    pass
+
+
 class PeppolServiceError(Exception):
     pass
diff --git a/modules/edocument_peppol/message.xml b/modules/edocument_peppol/message.xml
index 9b6263bdb5..9e7a0abbea 100644
--- a/modules/edocument_peppol/message.xml
+++ b/modules/edocument_peppol/message.xml
@@ -9,5 +9,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_invoice_party_peppol_required">
             <field name="text">The party "%(party)s" is not set up to send the invoice "%(invoice)s" via the Peppol network.</field>
         </record>
+        <record model="ir.message" id="msg_invoice_line_unit_price_not_negative">
+            <field name="text">In order to post the invoice "%(invoice)s" on the Peppol network, the unit price of the line "%(line)s" must not be negative.</field>
+        </record>
     </data>
 </tryton>