/includes/modules/payment/linkpoint_api.php
around approx line 877 or so you'll see this section of code
Add the new line as shown:
Code:
  function _doCapt($oID, $amt = 0, $currency = 'USD') {
    global $db, $messageStack;
    $_POST['captamt'] = preg_replace('/[^0-9.%]/', '', $_POST['captamt']);