"How to import(bring back into an online wallet) offline bitcoin for use in commerce:
IMPORTANT: WHEN IMPORTING FROM A PAPER OFFLINE WALLET, YOU MUST IMPORT THE ENTIRE AMOUNT OF THE OFFLINE WALLET. IMPORTING A FRACTION OF IT WILL CAUSE THE REMAINING BALANCE TO BE LOST FOREVER."
cant stress this enough.
Into bitcoin-qt.
Fortunately other clients (including blockchain.info) have a more sensible implementation. But to stress - read and understand how your client works.
In any case, you should import the full amount because it's not paper anymore once the private key hits an online system.