How can I export a single wallet from bitcoind?

To clarify, what you exported is a private key for a address. A wallet is a collection of addresses and their private keys.

The amount you think is associated with that address is definitely not the actual amount. Most likely, you have made a transaction in the past that spent some of those currencies and the rest ended up in a direction of change. When you make a transaction, the Bicoin exchange always goes to a newly generated exchange address that is part of your wallet.