Need Serious Help! Disallowed Key Characters.

[eluser]mindprojects[/eluser]
Hi guys,
i've developed my entire app following tha basic config of Codeigniter:

$config['uri_protocol'] = "AUTO";

Now i need to get the variable importo from this return url from a payment gateway:

http://127.0.0.1/backoffice/adria/bookin...0&nome=qwe&$BRAND=MasterCard

So i changed:

$config['uri_protocol'] = "PATH_INFO";

and enabled query strings:

$config['enable_query_strings'] = TRUE;

Once done, i got this error message:
"Disallowed Key Characters."

What to do? I'm getting mad..

PS: I found that the problem is in the $ ($BRAND) character...but also escaping that char in the $config['permitted_uri_chars'] nothing is going to change.