diff --git a/invoice.php b/invoice.php
index d8406eb..a84fc9d 100644
--- a/invoice.php
+++ b/invoice.php
@@ -32,6 +32,11 @@
 $desc = GETPOST('desc');
 $pay = GETPOST('pay');
 
+function fail($message){
+    header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error', true, 500);
+    die($message);
+}
+
 $sql="SELECT rowid FROM ".MAIN_DB_PREFIX."facture where facnumber='ProvPOS-$place'";
 $resql = $db->query($sql);
 $row = $db->fetch_array ($resql);
@@ -52,6 +57,26 @@
 	$now=dol_now();
 	$invoice = new Facture($db);
 	$invoice->fetch($placeid);
+    if($invoice->total_ttc<0){
+        $invoice->type= $invoice::TYPE_CREDIT_NOTE;
+        $sql="SELECT rowid FROM ".MAIN_DB_PREFIX."facture WHERE ";
+        $sql.="fk_soc = '".$invoice->socid."' ";
+        $sql.="AND type <> ".Facture::TYPE_CREDIT_NOTE." ";
+        $sql.="AND fk_statut >= ".$invoice::STATUS_VALIDATED." ";
+        $sql.="ORDER BY rowid DESC";
+        $resql = $db->query($sql);
+        if($resql){
+            $obj = $db->fetch_object($resql);
+            $fk_source=$obj->rowid;
+            if($fk_source == null){
+                fail($langs->transnoentitiesnoconv("NoPreviousBillForCustomer"));
+            }
+        }else{
+            fail($langs->transnoentitiesnoconv("NoPreviousBillForCustomer"));
+        }
+        $invoice->fk_facture_source=$fk_source;
+        $invoice->update($user);
+    }
 	if (! empty($conf->stock->enabled) and $conf->global->CASHDESK_NO_DECREASE_STOCK!="1") $invoice->validate($user, '', $conf->global->CASHDESK_ID_WAREHOUSE);
 	else $invoice->validate($user);
 	// Add the payment
diff --git a/langs/en_US/takepos.lang b/langs/en_US/takepos.lang
index 1910389..0c21425 100644
--- a/langs/en_US/takepos.lang
+++ b/langs/en_US/takepos.lang
@@ -19,4 +19,5 @@ Floor=Floor
 AddTable=Add table
 Place=Place
 TakeboxNecesary='TakeBOX' application required
-OrderPrinters=Order printers
\ No newline at end of file
+OrderPrinters=Order printers
+NoPreviousBillForCustomer=Cannot do negative bill without previous bill for current customer
diff --git a/langs/fr_FR/takepos.lang b/langs/fr_FR/takepos.lang
index fb2a457..aeec68b 100644
--- a/langs/fr_FR/takepos.lang
+++ b/langs/fr_FR/takepos.lang
@@ -19,4 +19,5 @@ Floor=Étage
 AddTable=Ajouter une table
 Place=Endroit
 TakeboxNecesary='TakeBOX' Application requise
-OrderPrinters=Commande imprimantes
\ No newline at end of file
+OrderPrinters=Commande imprimantes
+NoPreviousBillForCustomer=Impossible de créer une facture négative sans une facture précédente pour le client courant
diff --git a/pay.php b/pay.php
index 5479968..105a5a3 100644
--- a/pay.php
+++ b/pay.php
@@ -47,7 +47,7 @@ function addreceived(price)
 	$('#change1').html(received.toFixed(2));
 	if (received><?php echo $invoice->total_ttc;?>)
 		{
-		var change=parseFloat(received-<?php echo $invoice->total_ttc;?>);
+		var change=parseFloat(received-(<?php echo $invoice->total_ttc;?>));
 		$('#change2').html(change.toFixed(2));
 		}
 	}
@@ -60,9 +60,13 @@ function reset()
 	}
 	
 	function Validate(payment){
-        parent.$("#poslines").load("invoice.php?place=<?php echo $place;?>&action=valid&pay="+payment, function() {
-            parent.$("#poslines").scrollTop(parent.$("#poslines")[0].scrollHeight);
-            parent.$.colorbox.close();
+        parent.$("#poslines").load("invoice.php?place=<?php echo $place;?>&action=valid&pay="+payment, function(response, status, xhr) {
+            if(status == "error"){
+                alert(response);
+            }else{
+                parent.$("#poslines").scrollTop(parent.$("#poslines")[0].scrollHeight);
+                parent.$.colorbox.close();
+            }
         });
 		
 	}