The prayer of one who leads the people after the imam is delayed without being appointed as his substitute

Question:
He says: Honorable Shaykh, may Allah grant you success. If a person prays with the people after the imam is delayed, without the imam appointing him as a substitute, and then the imam arrives and is not pleased with his leading, is their prayer invalid?

Answer:
If the delay is such that it harms the ma'mum (those praying behind the imam), then it is permissible for him to pray with them. The one who is at fault is the imam who was delayed. The one who prayed with the people when the imam was delayed a long time is the one who did well, and the one who did wrong is the imam who was delayed. Yes, their prayer is valid in this case. However, if he prays with them before the time for the imam to arrive, that is a matter of concern. It is forbidden to lead the prayer before a regular imam without his permission or an excuse. Without his permission or an excuse, it is forbidden for one to pray with the people before the imam, and there is no excuse here; the time for the iqamah (the second call to prayer) has not come. Yes.