أو دفع في ظلمة إلى فقير ثم بان أنه أبوه أو ابنه فلا إعادة عليه
Translation: Or he gave to a poor person in darkness who later proved to be his father or son; then no repayment is required in that view.
This example concerns a genuine mistake of identity. The ordinary bar against paying a parent or child still stands.
The point under discussion is whether an already-made mistaken payment has to be repeated under the view of Abu Hanifah and Muhammad.