What is the ruling regarding someone who gave testimony, then forgot it and swore an oath to that effect, then retracted their oath and reinstated the original testimony they gave while a minor, and what is the expiation due for that?

If the first testimony was truthful, it is not permissible to conceal it if there is no one else to bear witness. Testimony is a communal obligation (fard kifaya) when there is someone through whom the right can be established, and its owner requests it, and the witness does not fear harm. Testimony may become an individual obligation (fard ayn) if there is no one else to establish the right except the witness, and its non-performance would lead to the loss of that right.

However, if the first testimony was given before puberty, you are not accountable for it concerning the right of Allah, because the pen is lifted from a child until he reaches puberty. Nevertheless, the child is responsible for compensation in cases of damage.

If the testimony was false and a judgment was rendered based on it, you are liable for compensation to the person against whom the judgment was made. If it was given after puberty, it is a false testimony and a grave sin, one of the greatest major sins, as is a false oath.

Your statement that you remembered nothing, then you retracted and testified to what you had previously testified to, is considered a retraction of testimony that invalidates it before judgment, but not after. If the witness retracts after judgment, the testimony is not invalidated, and the witness is liable for financial compensation and blood money (diya) to the person against whom they testified.

You must hasten to repent if you testified or swore falsely. There is no expiation for a deliberate false oath (yameen al-ghamūs) according to the majority of scholars. However, according to the Shafi'i school, expiation is required, which is more cautious. If your testimony resulted in the loss of a right for an oppressed person, you must compensate them for it.