Is a person's oath, "By Allah, I don't even have a single pound," while possessing larger denominations (one hundred pounds and a dollar), considered a breach of an oath and a lie, or a permissible prevarication, and what is the required expiation if it is a breach of an oath?

If the one swearing an oath firmly believes there isn't a single pound with him, or intends to deny the existence of what the questioner desires, then he does not break his oath and does not incur sin. This is because his wording can carry these meanings, and his intention aligns with them.