Is it permissible for the husband to retract his oath after prohibiting his wife by saying, "You are unlawful to me," if she buys from a specific woman, and what is the expiation for that?

The ruling on swearing to make one's wife unlawful varies depending on the husband's intention: If he intended divorce, then it is a divorce that takes effect if the condition is met, and he can take her back if it was not the third divorce. If he intended zihar (a pronouncement making one's wife unlawful, akin to declaring her as one's mother's back), then it is zihar, and the expiation for zihar is obligatory before intercourse. If he intended an oath or did not intend anything specific, then it is an oath for which expiation for an oath is obligatory, and he can retract the oath and do good after expiating. The lawful oath is an oath by Allah the Almighty.