Redeeming iTunes/Apps vouchers
are you able to stop redeeming the voucher and save it for another time? i.e. return to being charged on your credit card and save the remaining balance?
When you redeem a card onto your account then the card's value will be added to your account (a card can only be redeemded once, you can't put value back onto a card) - whilst you have a balance on your account that will be used for your purchases, your card will only be charged when your balance has run out.
How purchases are billed : http://support.apple.com/kb/HT5582
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.