Not allowing a specific & symbol in Codeigniter URL

Hi all,  I'm new to Codeigniter and stuck in a problem that I can't seem to figure  out at all.

The following URL results in an error which says: "The URI you submitted has disallowed characters."

Code:

http://example.com/index.php?/Paypal/success&tx=8T317791V7961045T&st=Completed&amt=20%2e00&cc=USD&cm=&item_number=

However, if I simply change the & symbol after 'success' to ? like below then everything works fine despite the fact that there are other & symbols in the URL as well. What is going on here? 

Code:

http://example.com/index.php?/Paypal/success?tx=8T317791V7961045T&st=Completed&amt=20%2e00&cc=USD&cm=&item_number=

I tried doing the following in Config.php but it didn't work:

PHP Code:

    $config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-\&'; 


OR 

PHP Code:

    $config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-&'; 


How do I make it so that my original URL is accepted?