Originally Posted by
Xandrios
What you could do is book the cheapest r/t fare, and not enter your FB details. Pay with anything else than creditcard, so KLM cannot just debit a surcharge, and you should be fine.
In many jurisdictions, a merchant could not debit your card without your authorisation. So if you have only authorised the amount of the ticket, KL could not unilaterally debit more from your card even if you happened to owe them money.