Well, suppose that you do give them the unencrypted file. They already have the encrypted file and presumably are aware of the encryption scheme you used, so (this could be totally wrong, as I don't know enough about encryption) they should be able to figure out the encryption key.
Any encryption scheme that is not worthless does not work that way. If you have the plaintext and the ciphertext (but did not get to choose the plaintext to be encrypted) then you are still no closer to getting the key.
