Sadly you can't chose which transaction you want to pay as first. If you made a bid on 2 characters and auction end at the same time and you dont have enough money, then first your money will be taken to pay auction on which you made your first bid . Then if you have enough money, it will be taken to pay your second auction.
I was in the same situation, and I got character that I didnt want.... What you can try to do is to wait till first auction is cancelled and then you can pay for the second auction. Im not sure how much time difference is between your bids...For example if your first auction was won at server save and second later, then you can wait till website will cancel your first purchase after 7 days and pay for the second character.
What happens if you don't have enough money?
If you lack the necessary funds, you have 7 days to pay the selling price. When paid, the character will appear in your character list after the next server save, and its location will be reset so it logs in its home temple.
If you cannot pay the selling price after the 7 days, the reserved coins will be subtracted from your account's coin balance and the auction annulled. The seller will be informed via email about the annulled auction and receives their 50 TC auction fee back.
Source