What I meant with "for accounting purposes", is that I booked ticket A and billed the customer. Any other trips within that time frame (such as my current round trip), are paid for by me.

Also, I still am not sure how it could be back to back ticketing, since ticket A is DEN-AMS; BRU-DEN and ticket B DEN-AMS-DEN. I will be returning from BRU on my first ticket, not from AMS.