buying tickets for a couple travelling to the UK

[ashakar]

I need to purchase return tickets for a Moroccan couple travelling to the Uk. I have found a flight for them, but the problem is the airline want to charge me in dirhams. I am paying by visa (barclays) and was wondering how much they will charge me for this transaction. I've looked on their website but couldn't find any relevant info. I hope someone can help. Thanks in advance.