Is the prayer of the one who is performing a voluntary prayer valid when he leads the one who is performing an obligatory prayer?

Question:
One of the brothers asks: He prayed in congregation with the Muslims, then some people came during the same obligatory prayer and said to him, "Pray with us." He had already prayed the obligatory prayer before that, and none of them was more proficient in recitation than he was. What is the ruling on his prayer?

Answer:
This is a question about a man who prayed with the Muslims and completed the congregational prayer, then some people who had missed the prayer came and asked him to lead them because he was more proficient in recitation than they were, or more knowledgeable about a good matter for which they asked him. Should he lead them?

Answer: Yes, he should lead them. It is established from Mu'adh (may Allah be pleased with him) that he used to lead his companions in the 'Isha' prayer after he had prayed with the Prophet (peace and blessings be upon him). He would pray the obligatory prayer with the Prophet (peace and blessings be upon him), then come and lead his companions in their obligatory prayer while he himself was performing a voluntary prayer. There is no harm in that; because this is a legitimate religious benefit, and he is helping them to obey, teaching them jurisprudence, and instructing them; because they follow his example in prayer, and perhaps he will advise them and direct them to good.
So the conclusion: There is no harm in that. Likewise, the Prophet (peace and blessings be upon him) in some of the prayer of fear, led his companions in two rak'ahs of the prayer of fear while traveling, then led another group of them in two rak'ahs. So the first was an obligatory prayer for him, and the second was a voluntary prayer for him (peace and blessings be upon him), because he was the most knowledgeable of them and the most deserving of leading them (peace and blessings be upon him). Therefore, he led them in both prayers: the first as an obligatory prayer, and the second as a voluntary prayer for them.
And likewise, in one of the opinions regarding the Farewell Pilgrimage, he led those with him in Makkah in the Zuhr prayer on the Day of Sacrifice, then returned and led those in Mina who had not entered and were waiting for him. He led them in the Zuhr prayer, which was a voluntary prayer for him and an obligatory prayer for them. This is evidence that there is no harm in repeating the prayer by the imam, or by someone other than the imam, for the sake of the great benefit in his being the most knowledgeable of them, the most proficient in recitation, and the like.