Join Date: Aug 2006
Location: MUC (home), DUS (office), XXX (customer)
Programs: LH, AB, SPG, CC, Sixt, EC
Posts: 6,332
Do it once, nothing will happen. If you request fees & taxes back for the return, they will start to reprice the ticket. So just leave it the way it is, buy the return, fly the one way.
It is obvious though that the flight that you actually WANT is the one that you need to book as first leg! If it is the return and you leave the first segment unused, they will cancel the entire ticket.