Seems like people always forgot to filter the (-) minus sign.
Anyway, I think this should fix the problem.
Open your trade.php
find this code
PHP Code:
$adoptwanted = $_POST["adoptwanted"];
$adoptgiven = $_POST["adoptgiven"];
$cashgiven = $_POST["cashgiven"];
$message = $_POST["message"];
Just below that, add
PHP Code:
$cashgiven = abs($cashgiven);