FIX Withdraw request should not be possible if amount is negative.

htdocs/compta/facture/prelevement.php

 <?php
 /* Copyright (C) 2002-2005	Rodolphe Quiedeville	<rodolphe@quiedeville.org>
  * Copyright (C) 2004		Eric Seigne				<eric.seigne@ryxeo.com>
- * Copyright (C) 2004-2015	Laurent Destailleur		<eldy@users.sourceforge.net>
+ * Copyright (C) 2004-2016	Laurent Destailleur		<eldy@users.sourceforge.net>
  * Copyright (C) 2005-2012	Regis Houssin			<regis.houssin@capnetworks.com>
  * Copyright (C) 2010-2014	Juanjo Menent			<jmenent@2byte.es>
  *
@@ -482,6 +482,8 @@ if ($object->id > 0)
 
 	// Add a withdraw request
 	if ($object->statut > Facture::STATUS_DRAFT && $object->paye == 0 && $num == 0)
+	{
+	    if ($resteapayer > 0)
 	    {
     		if ($user->rights->prelevement->bons->creer)
     		{
@@ -499,6 +501,11 @@ if ($object->id > 0)
     		}
 	    }
 	    else
+        {
+            print '<a class="butActionRefused" href="#" title="'.dol_escape_htmltag($langs->trans("AmountMustBePositive")).'">'.$langs->trans("MakeWithdrawRequest").'</a>';
+        }
+	}
+	else
 	{
 		if ($num == 0)
 		{