He wiped over see-through socks, then he realised that that is not permissible. What should he do?

Most of the scholars are of the view that it is not permissible to wipe over the socks unless they are thick and the foot or part of it cannot be seen through them.

Others are of the view that it is permissible to wipe over the socks even if they are thin. That was narrated from ‘Umar and ‘Ali (may Allah be pleased with them), and was regarded as more correct by Shaykh al-Albani and Shaykh Ibn ‘Uthaymin (may Allah have mercy on them).

Please see the answer to question no. 228222 .

There is no hadith from the Prophet (blessings and peace of Allah be upon him) to clarify this issue; rather it is a matter that is subject to ijtihad, and with regard to such matters, if the Muslim follows one of the two views based on a fatwa that he heard, then he finds out that the other view is more likely to be correct, then he should follow the other view, but the acts of worship that he did based on the first view are valid and he does not have to repeat them.

Al-Hajawi (may Allah have mercy on him) said regarding one who realises that the qiblah is in a different direction from what he thought based on his own efforts to work it out:

He should pray facing the second direction, but he does not have to make up the prayers that he offered facing the first direction, which he thought was correct. End quote.

Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in his commentary:

The words: “and he should pray facing the second direction and does not have to make up the prayers that he offered facing the first direction” mean: when he realises that he was mistaken, because he thought the first direction was correct, based on doing what was expected of him of trying to work it out, and whoever makes the effort that is expected of him does not have to repeat acts of worship that he did on that basis if he realizes that it was wrong, because if we say that he has to repeat those acts of worship, it would mean having to do each act of worship twice.

So if he prayed Zuhr facing towards the north, believing it to be the qiblah on the basis of his best efforts to work it out, then at ‘Asr time it became clear to him that the qiblah is towards the south, then he does not have to repeat Zuhr, because he offered that prayer based on what he thought was correct after doing what is enjoined of seeking to reach a conclusion (ijtihad), and the second ihtihad does not nullify acts of worship that were done on the basis of the first ijtihad.

Similarly, in issues of research, if a man develops a view on the grounds that this is what the texts indicate, then after further research and discussion he comes up with a view that is different from his first view, he does not have to undo a judgement, if he judged between people on the basis of his first view, and he does not have to undo his fatwa. So he does not have to go to the one to whom he gave a fatwa based on his first view and say: I gave you a fatwa, then I realised that that fatwa was wrong. That is because the first fatwa was based on ijtihad, so it cannot be rendered invalid by the second ijtihad.

The reason why we say that the first fatwa should not be undone is: as he thinks that he was mistaken in his first ijtihad, on the basis of which that fatwa was given, it is also possible that he could be mistaken in his second ijtihad. Perhaps the first one is the one that was correct, when he thought that the second one was correct. Hence the scholars said: ijtihad cannot be undone by further ijtihad.

It was narrated from ‘Umar (may Allah be pleased with him) regarding the issue of al-himariyyah [a famous issue having to do with shares of inheritance, which is when the deceased leaves behind a husband, a mother, brothers through the mother and full brothers], that he ruled that the full brothers do not receive any share of inheritance. Then later on the same scenario happened again, and he ruled that they could be included. He was asked about that, and he said: That [the first ruling] was based on our ijtihad at the time, and this [the second ruling] is based on our ijtihad now. And he did not undo the first ruling."(Al-Sharh al-Mumti‘  2/288).

Based on that, your previous prayer is valid, because it was based on a fatwa issued by a scholar who thinks that is permissible.

And Allah knows best.