def _amount_residual(self, cr, uid, ids, name, args, context=None):
        result = {}
        for invoice in self.browse(cr, uid, ids, context=context):
            result[invoice.id] = 0.0
            if invoice.move_id:
                for m in invoice.move_id.line_id:
                    if m.account_id.type in ('receivable','payable'):
                        result[invoice.id] += m.amount_residual_currency
            #prevent the residual amount on the invoice to be less than 0
            result[invoice.id] = max(result[invoice.id], 0.0)            
        return result