Is money considered ill-gotten (haram) due to past theft, and is there an expiation for that, given repentance and the small amount stolen?

The thief must return the stolen item if it is still intact. Otherwise, if it is a fungible item, he must return its equivalent. If it is a non-fungible item, he must return its value. The proof for this is the saying of the Messenger of Allah (peace be upon him): "On the hand is what it has taken until it renders it back."