I have a guest who is asking what amount is outstanding on his booking (he is due to arrive next month). Unfortunately as a host I am not able to check this. Right? He says he cannot find it on his Airbnb app.
You are right, you're not able to check this- Airbnb handle all payments/transactions for the guest and then hold payments to you until 24 hours after your guest checks-in.
You should advise your guest to contact Airbnb directly to answer his question if he can't see the information he needs on his reservation or correspondence with Airbnb.