it's unfortunate that you've made a bummer bid that might damage your finances. I can understand why you would wish for a withdrawal option for your bids. as a newcomer to BB myself I also understand how you feel in regards to navigating the site. It is confusing at times.
Unfortunately, what's done is done. Hopefully it won't be something that will hurt your team so badly that they will go bankrupt. If it's that bad, you could relist the players and try and recoup the money that you lost. the good thing is that you've learned from your mistake and won't overpay on a player ever again :)
Hopefully you and I will get used to the site and the game and be able to go to the tops of our leagues quickly :)
Cheers!