Thanks for the help from Eric, I was able to resolve the issue. The support issue has also been raised with the Instamojo plugin
In the Instamojo plugin change the following
Open the file – Charitable_Gateway_Instamojo.php
and change the following line
FROM
$api_data[‘redirect_url’] = $return_url . “?donation_key=” . $donation_key;
TO
$api_data[‘redirect_url’] = $return_url . “&donation_key=” . $donation_key;