diff --git a/htdocs/compta/facture/card.php b/htdocs/compta/facture/card.php
index c299c1d1de5..cd436543079 100644
--- a/htdocs/compta/facture/card.php
+++ b/htdocs/compta/facture/card.php
@@ -1570,7 +1570,7 @@ if (empty($reshook))
 			setEventMessages($langs->trans('ErrorFieldRequired', $langs->transnoentitiesnoconv('Type')), null, 'errors');
 			$error ++;
 		}
-		if ($prod_entry_mode == 'free' && empty($idprod) && (! ($price_ht >= 0) || $price_ht == '') && $price_ht_devise == '') 	// Unit price can be 0 but not ''
+		if ($prod_entry_mode == 'free' && empty($idprod) && (($price_ht < 0 && empty($conf->global->FACTURE_ENABLE_NEGATIVE_LINES)) || $price_ht == '') && $price_ht_devise == '') 	// Unit price can be 0 but not ''
 		{
 			setEventMessages($langs->trans("ErrorFieldRequired", $langs->transnoentitiesnoconv("UnitPriceHT")), null, 'errors');
 			$error ++;
@@ -3047,10 +3047,10 @@ else if ($id > 0 || ! empty($ref))
 		}
 	}
 
-	// Confirmation de la validation
+	// Confirmation of validation
 	if ($action == 'valid')
 	{
-		// on verifie si l'objet est en numerotation provisoire
+		// we check object has a draft number
 		$objectref = substr($object->ref, 1, 4);
 		if ($objectref == 'PROV') {
 			$savdate = $object->date;