Does one who prayed alone, believing the congregation had finished, repeat the prayer with the congregation?

Question:
May Allah reward you. A man prayed the 'Asr prayer alone, believing that the people in the mosques had finished the prayer. When he finished his prayer, he was surprised to find that the prayer was being established. Does he pray with the congregation? Or what does he do?

Answer:
This man prayed while excused. On the day he prayed, he was excused because he thought the people had already prayed. So he fulfilled the obligatory prayer. He fulfilled the obligatory prayer, and praise be to Allah, based on his ijtihad (independent legal reasoning). Yes.