I just did a bit of experimenting and I put my Auftrag (PNR) code into both the German and English language links above, and the cancelled ticket appeared immediately. You get a big red S, which stands for Storniert (cancelled). If you look right at the bottom, you can see the voucher code there after the words STORNO GUTSCHEIN FV (I presume the FV means Fahrkarte Verkauf). The following seven letters and numbers is your voucher (but not the hash # symbol, of course). So, you should be fine.