We've had a business account with paypal for about 2 years now, and it's the first time that we have encountered this type of enquiry from one of our members.
I have since checked with some other members to our site, and it appears that they are able to log into their accounts and use their paypal balance. Therefore, the enquiry raised must need dealing with by paypal and the user concerned, and is not one to do with nBill.
Thank you for answering my post so quickly, it's much appreciated.