What expiation is due if one makes a pronouncement of Zihar concerning one's female slave according to the view of Ahmad ibn Hanbal?

Primary text

It is narrated from Ahmad that the one who pronounces Zihar concerning his female slave owes the expiation of Zihar. However, Abu Bakr stated that if the expiation of Zihar were due, it would mean the Zihar was valid, which contradicts the main position. Therefore, only the expiation of an oath is due because it is a prohibition of something permissible to him as property, similar to prohibiting the rest of his wealth. Evidence for the expiation of an oath is the narration that the Prophet (peace be upon him) prohibited his female slave, and Allah commanded him to expiate his oath.